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Two Terminal Moraines in Mission Valley, MT


The Glaciation of Mission Valley

The Cordilleran Ice Sheet invaded northwestern Montana multiple times during the Pleistocene. The ice sheet overtopped mountain ranges and widened pre-glacial river valleys in northern Montana, Idaho, and Washington. Alpine glaciers descending from high cirques also reached the floors of many valleys. Mission Valley, located at the southern end of the Rocky Mountain Trench and south of Flathead Lake, was invaded by both the Flathead Lobe and alpine glaciers flowing west from the Mission Mountains, orthogonal to the ice sheet.

The CIS can be divided into several valley-filling lobes, from west to east: Puget (WA), Okanogan (WA), Columbia River (WA), Pend Oreille (ID), Purcell Trench (ID), Thompson River (MT), and Flathead (MT).

Three tills, associated outwash gravels, and varved silt-clay beds of Glacial Lake Missoula comprise the valley fill. The age of Mission Valley's glacial deposits remains in question. Are all three tills late Wisconsin (Pinedale glaciation, ~20 ka) or are some pre-late Wisconsin (Bull Lake, ~130 ka)? Was there one Glacial Lake Missoula or two? How far beyond the last-glacial Polson moraine did earlier ice advance? In this article, I review the known glacial history of Mission Valley and explore new field-based discoveries. A new map of the valley's surficial geology is in the works.

The Missions. View to the southeast over Big Arm bay (Flathead Lake) to the Mission Mountains and Mission Valley. Pablo Reservoir is seen on the valley floor at center right. The Jocko River Valley is in the hazy distance at right and marks the southern end of the range. I took this photo from the road to Blacktail Ski Area above Lakeside, MT in 2021.

Recessional moraines left by retreating ice. Glaciers form terminal or end moraines at their maximum-advanced positions (dark blue line). Less obvious are recessional moraines left behind during deglaciation (purple lines). Recessional moraines mark temporary pauses of a glacier in retreat. Recessional moraines are often less prominent than end moraines. The figure above is from Wyoming's Wind River Range and is used here to illustrate the recessional moraine concept. The record of ice advance and retreat in the Mission Valley, MT is more complicated, but generally follows the same pattern. I drew this figure.

A testable hypothesis. Did the Flathead Lobe terminate at the Polson moraine (red) or the Mission moraine (blue)?

Mission Valley. The Mission Valley is located in northwest Montana south of Flathead Lake, west of the Mission Mountains, and entirely inside the boundaries of the Flathead Indian Reservation. The towns of Polson and Dixon mark the north and south ends of the valley. Source: Fremont (1848).

Mission Valley geography. Source: Levish (1993).

Did the Flathead Lobe advance south of Polson?

Because the Mission Valley does not contain ice-contact deposits, there is no record of a grounded Flathead lobe extending into the valley south of the Polson moraine...The stratigraphic record does not support the postulated multiple Pleistocene advances of the Flathead Lobe. - Daniel Levish (1997)

If ice had ever extended to the south end of the Mission Valley, more than 1000 feet of ice would have sculpted the Valley View Hills. Similarly, ice would have reached nearly to the summit of the Moiese Hills. The morphology of these hills does not record glacial erosion or deposition. - Levish, Ostenaa & Klinger (1993)

The statements above are key findings by Dan Levish in his dissertation completed at University of Colorado Boulder under Dr. Peter Birkeland (Levish et al. 1993, Levish 1997). While I agree that classic constructional morphology and hummocky, boulder-strewn surfaces are largely absent from moraines in Mission Valley, I disagree that 'no record' of ice advance south of the Polson moraine exists. Geologists a century ago noted morainal ridges crossing the valley floor between Ronan and St. Ignatius and I find them mappable today. I am not of the opinion that more recent work is always better than the older stuff.

Two major glacial pauses are preserved in Mission Valley. The younger advance (MIS 2) terminated at the Polson moraine (3487') just beyond the south end of Flathead Lake. The older advance (early MIS 2, MIS 4, or MIS 6?) produced the Mission moraine, which terminated near St. Ignatius more than 30km south of Polson. There are hints of an even older, more southerly terminus as well. Its retreat left behind a set of low-relief, horseshoe-shaped ridges that cross the valley and are composed of stratified bouldery-gravelly-clayey material (diamict). Sedimentology of the diamicts suggests sediments were rained from the base of a floating ice shelf through water of significant depth onto a soupy lake bottom (i.e., Dan Levish's model modified from Tope and Powell). The plan-view shapes of the ridges - contiguous, nested horseshoes - suggest deposition by grounded ice (a typical glacier), but the matrix-supported and stratified diamict suggest ice floating on a lake.

Former lakebed. Glaciers that sculpted the Mission Valley came in two forms. The larger Flathead Lobe of the Cordilleran Ice Sheet advanced south out of Canada and down the axis of the valley, while smaller alpine glaciers, sourced in cirques, descended the steep west flank of the Missions at right angles to the axis. End moraines of alpine glaciers lie below the uppermost strandlines of Glacial Lake Missoula and often appear to have been chopped off just before reaching the valley floor (Steere 1895). My photo taken kind of near my house.


Long history of geological investigation in Mission Valley

Native peoples certainly understood the geology of northwest Montana, even if written accounts are scarce. According to Blixt (1931),

...before the white man, the Indian showed himself a practical geologist: deposits of pipestone-clay and of obsidian and flint suitable for arrow heads were worked by him. The Snake Indians, in calling the territory which is now Montana...'land of the mountain', gave evidence of their observation of the broken and folded character of its surface.

The explorer David Thompson (c. 1812), Lieutenant A.J. Donelson (1853), Lieutenant Rufus Saxton (1853), and naturalist J.G. Cooper (1853-55) came before, but it was geologist T.C. Chamberlain who first formally investigated glacial deposits in Mission Valley. In 1881, John Wesley Powell, the newly appointed Director of the U.S. Geological Survey, tasked Chamberlain with mapping the entire system of Pleistocene glacial moraines from The Dakotas to the Atlantic coast - a monumental challenge (USGS 1883, 1889). His first draft was published in 1883 at a scale of 1 inch = 150 miles and covered a broad swath from North Dakota to Cape Cod. Unfortunately, Montana's glacial geology was left off the map.

Through the 1880s, Chamberlain would continue to work out the contours of the former ice margin, defining what would eventually become known as the 'Laurentide Ice Sheet' (USGS 1889, Chamberlain 1895, Flint 1943). Its western edge butts up against the eastern entrance of Glacier National Park.

This region...was doubtless during a recent geologic period sheeted over with ice in a manner which finds a counterpart in the present condition of Greenland. This conclusion - and a sim­ilar one that has been reached with respect to certain portions of Europe - rests upon a vast mass of circumstantial evidence so clear and convincing when fully understood, that it may be regarded as one of the most wonderful and pleasing examples of inductive reasoning, and one of the best established that the whole range of modern science affords.

Despite dedicating several years to his work out West, Chamberlain appears to have drawn no wage from federal coffers for it. In 1880, the USGS lists him as one of many "Expert Special Agents" working on a per diem of $5.00 per day. In 1889, Chamberlain confirms that while his assistants were fully supported for their work along the glacial margin, he drew only a professor's salary from the University of Wisconsin and later from the University of Chicago.

Upham and Leverett have been employed continuously in the work of the Survey throughout the year; all others, myself in­cluded, have worked upon a per diem basis, in connection with other employment in the main educational.

Way out east. The Mission Valley is located in northwestern Montana at the south end of the Rocky Mountain Trench, where the Flathead Lobe of the Cordilleran Ice Sheet terminated. The valley extends south from the southern shoreline of Flathead Lake at Polson to the Ravalli Hill divide south of St. Ignatius. The Mission Mountains and Flathead River form the valley's eastern and western boundaries, respectively. During the last glacial, much of western Montana was inundated by Glacial Lake Missoula, including Mission Valley. Its about as far east as you can go and still be talking about the Missoula Floods, or for that matter, the Cordilleran Ice Sheet. Map drawn by Waitt (1980) and modified by Levish (1997).

Another University of Chicago professor, R.D. Salisbury - or possibly G.H. Garry and E. Blackwelder, the geologists in his charge - briefly visited the Flathead and Mission Valleys near the close of the 1800s (Salisbury 1901),

The Flathead valley contained a great glacier which advanced southward beyond the southern end of Flathead lake, the southern limit of the ice was not traced out this season. In the latitude of Kalispell, the ice of this valley was about 3000 feet thick, as shown by the height to which the west face of the Kootenai Mountains [those of Glacial National Park] east of the valley, was glaciated. The general direction of ice movement in this valley at Kalispell...was south-southeast. The main body of the ice, therefore, came down the valley from the north northwest, and not from the mountains immediately east or northeast, though the main glacier was reinforced to some extent by ice from this direction. The source of ice which moved to the south-southeast was not determined. One lobe of the Flathead glacier moved southwest up the valley of Ashley Creek (the valley followed by the Great Northern Railway west of Kalispell), and another advanced westward a short distance beyond the end of the west arm of Flathead Lake.

Oregon Territory in 1849. Rivers, lakes, travel routes, and the general contours of mountain belts are shown on this 'Map of the Oregon Territory From The Best Authorities'. Flathead Lake and 'Lake Notre Dame' (Swan Lake) shown at right. Mission Valley in red box. Excerpt from Wilkes (1849, 2nd edition).

Railroads pioneer a route west. Excerpt of an 1860 map from the Stevens Pacific Railroad Survey (1853-1855) 'Milk River to the crossing of the Columbia River'. Mission Valley indicated by the red box. The 'Trading Post' is Fort Connah. No one was thinking about glacial margins at the time. David Rumsey Collection.

Watersheds out west. 'Flathead Lake' appears on the 'Jocko Indian Reservation' in this curiously colorful map of 'drainage districts' by USGS (1891). No glacial margin shown. Source: USGS 11th Annual Report Part II, Plate LXIX.

Flathead River in 1853. Lithograph by the expedition artist Gustavus Sohon depicting the 'View of the Clark's Fork and the ridge of mountains, south of the Flathead Lake, looking eastward'. The artist was sitting on a hill a couple miles south of Sloan's Bridge, across the river from Moiese Valley. Low treeless hills in the middle distance are the southern part of Valley View Hills, Round Butte, and northern part of Moiese Hills. The Mission Mountains are on the horizon beyond. Source: Print 65, 12th Volume of 1860 War Department Reports of Explorations and Surveys.

In 1889, Montana became a state. In 1899, the University of Montana established a Biological Station on Flathead Lake under the leadership of ecologist Morton J. Elrod. Soon after, scientists from collaborating universities began meeting there to poke about and author papers on the lake's biology and geology. In 1903, Professor Elrod writes,

Mission Valley is undoubtedly glaciated...At the lower end of the valley, near St. Ignatius Mission, large boulders lie high on the hills, while there are ridges and valley plainly morainal...The southern end of Mission valley has a much larger moraine than that at the foot of Flathead lake...The entire Mission Valley is made from glacial material, with a high morainal deposit at both the northern and southern ends...The large moraines previously mentioned extend from east to west, at right angles to the range. They are many miles in extent, much larger than any [glacial] drift from canyons.

Fred D. Smith, who briefly served as Professor of Chemistry and Geology at University of Montana (and Montana State), recognized that the modern lake, confined by the Polson moraine, is but a remnant of the much larger Pleistocene Glacial Lake Missoula (Smith in Elrod 1899). The ever pragmatic Smith, upon noting his university's inability to build a world class geology program (same as today), left his teaching position for one as a Mining Engineer in the gold rush boomtown of Sumpter, OR.

It is but the remnant of a lake that in [earlier] times occupied this valley throughout its whole extent. The great level plains on either end of the lake are the beds of sediment deposited in the former lake, and show by the character of their soils that the lake was a large and quiet body of water...The lake may have occupied different levels in its present position...a larger embankment along its southern end...may be of morainal origin...At the end of the Swan River Valley near the location of the Biological Station are to be seen many rounded hills which are probably morainal in origin. On the slopes of the Mission Mountains...are found many evidences of glacial action in form of smoothed rocks, post-glacial gorges and stream courses, glacial scratches, etc...There is no doubt but that glacial agencies have materially affected the history of the lake both in its present and in its older form.

Morton's canoe. Plate 16 in Elrod (1899) showing "Canvas boat and plankton outfit of Montana Biological Station at Swan Lake, Montana, August, 1900. At the outlet of the lake looking into Swan River. Swan Mountains in the distance to the right."

Smith describes the Flathead Lake's outlet through the Polson moraine entering a bedrock gorge, where SKQ/Kerr Dam sits today and where the Lower Flathead River begins,

About a mile from the lake there is a large bank of clay through which the river has cut....the moraine mentioned. At the river bank it has been cut and eroded by the wind and rain. The bank is abrupt and steep, the clay clinging together so as to form cliffs, some ending in sharp pinnacles. Below the clay is the bed rock...The river has done some cutting through the solid rock bed, but not much...Above and below this place the river is a beautiful sheet of foam, with several small falls. It is as beautiful a rapid as one usually sees.

In 1890, the geologist G.E. Culver traveled from Columbia Falls south through Mission Valley to meet the Northern Pacific Railroad at Dixon. He was returning from a 370-mile journey that began at Simms, MT under the leadership of Lieutenant George P. Ahern. The team pioneered a new packtrain route across "Ahern Pass" through the heart of what in 1910 would become Glacier National Park. Culver was a teacher in Wisconsin and father of Harold E. Culver, a Washington State University geology professor.

Home of grizzlies. Mt. McDonald (9820'), the highpoint in the Missions, hosts a permanent snowfield. The summit stands nearly 7000 feet above the valley floor. Nearly the entire range is today designated wilderness (Tribal on west side, USFS on east).

William Morris Davis, a Harvard professor and the so-called 'Father of American Geography', first visited the Mission Valley in 1912 by train, returning in 1913 to conduct field work. Davis (1916) is remarkably readable, describing the north-to-south progression of glacially-sculpted landforms in the Mission Mountains. The answers are not all there, but it's a great early contribution.

Mission Mountains geomorphology. The Flathead Lobe of the Cordilleran Ice Sheet, which filled the Flathead Lake basin during the last glacial, is depicted here terminating at the Polson moraine. The geomorphology of the Mission Mountains changes systematically from north to south. The crest of the range rises to the south as the elevation of the valley ice drops. Alpine glaciers descended all the way to the valley floor beginning at about Ronan. Davis uses 'plains' in the same way F.D. Smith does, to describe the gently-sloping surface of lake bottom silts. Figure modified from Davis (1916, Fig. 2) by me.

Joseph T. Pardee of the USGS under Alden was another early geologist detailed to Montana (Pardee 1910, 1942). Recently unearthed field notebooks reveal several important things. One, he was working in the Pacific Northwest as early as 1903. Two, Pardee, not Bretz, may have been first to interpret the Channeled Scabland as a flood-formed landscape. Three, he placed the terminus of the Flathead Lobe south of Post Creek in 1910, but moved it north to Polson by 1942.

South of Polson. USGS geologist Joseph T. Pardee places the terminus of the Flathead Lobe well south of the Polson moraine in his 1910 map (Pardee 1910, Fig. 4).

Not south of Polson. By 1942, Pardee moves the terminal moraine to Polson, at the south end of Flathead Lake (Pardee 1942, Fig. 3).

The 1915 'Guidebook to the Western United States' by M.R. Campbell (Campbell et al. 1915) provides a town-by-town description of the geology as seen from the Northern Pacific Railway. The popular 'railside' guide, a precursor to 'roadside' guides, provides an early description of shorelines etched into hillslopes of the Missoula, Clark Fork, Bitterroot, Jocko, and Mission Valleys (p. 135-135). Figure 29 on p. 134 closely resembles one in an earlier article by Pardee (1910, Fig. 2); the two USGS men surely talked. Campbell draws the southern terminus of the Flathead Lobe far south of Polson, near the latitude of Mt. McDonald,

It is probable that the glacier occupying the Flathead Valley reached at its greatest extension nearly or quite to the place now occupied by the town of Dixon, but there is no evidence that it passed farther down the valley.

Mission moraine south of Ronan. As early as 1916, the Campbell places a glacial moraine - the Mission moraine - in the southern part of the valley. His linework depicts its terminus between the Moiese Hills and the western flank of the Missions. Its snout is nearly at the latitude of Mt. McDonald (Campbell 1916, Sheet No. 19).

Shave ice at Dixon. Campbell's railway guide (1915) shows the Flathead Lobe terminating near the railway near Dixon, MT, where the glacier touches the tracks.

Times change. Two maps drawn a few years apart show the Flathead Lobe terminus at Polson (left, Atwater 1983) and later at St. Ignatius (right, Atwater 1986).

Montana geologists were quite familiar with Glacial Lake Missoula prior to 1900. An early account of a vast "co-glacial lake" is found in a curious thesis by F.A. Steere (1895), a student of University of Wisconsin professors C.R. Van Hise (geology) and E.A. Birge (limnology).

I shall proceed to designate this ancient co-glacial lake as Lake must have been at one time nearly 600 feet in depth...Overlooking the City of Missoula is a mountain whose peak is some 1000 feet above the valley. This is known locally as Mt. Jumbo. Six hundred feet up this mountain side and thus 4100 feet above the sea are the shore marks of the Co-Glacial lake.

Another thesis by University of Montana paleontology student, Earl Douglass (1899) mentions the lake and credits his instructors Bailey Willis (USGS), M.J. Elrod (U of MT), O.J. Craig (Princeton), F.D. Smith (Montana State), and W.R. Scott (Montana State).

In the Missoula and Bitter Root valleys on the mountain sides and along the foot hills are level lines or small terraces, evidently shore lines, formed by the dashing of waves against the mountain sides. These can be nicely studied around Missoula. The University buildings stand at the foot of a steep mountain slope, and on this slope about sixty of these lines can be counted, beginning near the foot and extending upward, perhaps a thousand feet. The same is seen on Mt. Jumbo, and on the hills north of Missoula. They are also found along the Bitter Root. Prof. Elrod says they are still plainer in the region of Flathead Lake. These lines are always level and keep at the same altitude along mountains of hard quartzyte and hills of loose gravel. It is difficult to see how these lines can be preserved so long where the material is soft and incoherent; and one is led to believe that the water cut with comparative rapidity through its barrier in geologically recent times. When the water reached its highest point it remained for a long time at or near the same level. Prof. Bailey Willis of the U. S. Geological Survey who has visited this region suggests the idea that this was a Pleistocene lake dammed by a glacier. In the Missoula valley are soft laminated clays...The lake beds lie in contact with all the older formations that occur in the western part of the state - the Archaeon, Algonkian, Cambrian, Devonian, Triassic, Jurassic, and Cretaceous. The newer bed lie unconformably on the older - at least in many places - and in the interval between the deposition of the two the [Tertiary] White River beds were tilted and carved into hills and benches and valleys, and the waters covered them again, after nearly all vertebrate life had changed, and a new deposit was made with remains of a later fauna entombed in it.

Photo of "Co-glacial Lake Powell" shorelines from Steere (1895) taken from Waterworks Hill over W Greenough Drive. View to east. The "L" on the hillside is just out of frame to the left.

Happy traveler. USGS geographer-geologist-photographer turned railside guidebook author, Marius R. Campbell, in Glacier National Park in 1913. USGS photo.

Davis scours the countryside. According to Davis, two ice tongues widened pre-glacial valleys in northwest Montana (1920, Fig. 1). He believed glaciers occupied all of Flathead Valley and Mission Valley well south of Polson. Glacial ice is also shown descending the Purcell Trench and along the Clark Fork Valley to Paradise, MT. Its a controversial interpretation, but one with which J Harlan Bretz agreed. Go see for yourself: Take the drive from Dixon to Thompson Falls through this remarkable canyon and you might leave similarly convinced.

In 1920, W.M. Davis published a second article containing a map that clearly shows ice at the southern end of the Rocky Mountain Trench (Mission Valley) and well advanced into the Purcell Trench (Clark Fork Valley). Ice flowed a considerable distance up the Clark Fork.

A soil survey by DeYoung and Roberts (1929) contains an excellent full-color map of Mission Valley. The report describes high-fertility soils and a valley with every promise to support farms and ranches well into the future. The authors noted low morainal ridges spanning the valley floor far south of Polson,

The surface relief becomes rather rolling north and west of Ronan bordering Mud Creek and also south and east of Round Butte....A large part of the ridge between Spring Creek and Mud Creek is gently rolling...South of Crow Creek and north of Charlo is another gravelly ridge or moraine...South of Charlo flat and Ninepipe Reservoir the land slopes rather sharply toward Post Creek. This gravelly slope parallels the streamcourse and appears to be another distinct glacial moraine.

The soil crew mapped and described stony soils formed on 'glacial till' and 'glacial morainic ridges' unrelated to tills left by alpine ice along the mountain front,

Most of the parent materials [for the McDonald Series and certain Crow, Polson, and Post variants] were transported and deposited chiefly during the recession of the large valley glaciers which completely filled the [lower] valley of Flathead River...As the valley glaciers receded, they left numerous terminal moraines.

Sultan of scabland. Dr. J Harlan Bretz (right) ranged widely during his long career as a field-focused geologist and professor. Bretz visited northwest Montana in 1928 and 1929. Photo from Intermountain Motorist (September 1928) taken during an excursion to Grand Coulee and vicinity arranged for Princeton University's Geology Department. "At the high spot of the road, the caravan stopped and the argument started." Miner and publisher Lyndon King Armstrong of Spokane (left) organized the outing. Bretz and Armstrong were both long time residents of Wisconsin prior to making their way west.

J Harlan Bretz passed Dixon, MT in 1928 by train and returned the following year for a closer look at the Glacial Lake Missoula silts and various gravels.

From NPRR train, August 17. No records of that lake seen north of Dixon. But here appear both the silts and the faint shorelines. Only a barren soil-mantled slope or one only grassed can show these shore lines; they are so faint. And bushes, shrubs and trees north of Dixon cover the slopes. The silts should be studied in detail and compared or contrasted with the flood silts. As seen from the train, they are much thicker and their terraces abut rather sharply against the higher slopes. No silt sections seen on these higher slopes. Instead, the cursory examination possible seemed to indicate gravelly and bouldery debris as the slope mantle. A few sections glimpsed. Silt seems to be much more uniform in the texture, and very well stratified with horizontal, uniformly dimensioned layers. No pockets of gravel, no lenses, no undulations, no sporadic boulders or cobbles seen. But, of course, this hasty glimpsing is inadequate for judgement. See what Pardee, Davis, Campbell have said about these silts. See if Alden found the flood characters.

Having completed his 1929 field visit, Bretz wrote to his friend and promoter, Thomas Large of Spokane, and disputes the interpretations of Alden and Campbell,

Last summer Alden told me that he thought [William Morris] Davis wrong in introducing glacial ice into Clark Fork valley near Thompson Falls [i.e., Davis 1917]; that what Davis had taken for moraine was simply sand dune...As a result of our work this summer I shall take sharp issue with Alden...Davis is right…You will remember that Pardee described some high level deltas in Lake Missoula and the Campbell followed him in this interpretations. Davis says that these are probably high lying fragments of lateral moraine. Pardee did not climb to any of these, Campbell certainly did not...We did!...we found more than Davis saw in the way of evidence for ice in this great [Clark Fork Valley] trough. We found that a lobe had come southward down the Little Bitterroot across Camas prairie clearly into the Flathead valley at Perma. We found also that another ice tongue had pushed down along Lynch Creek to Plains, and actually entered the Clark Fork valley at this place. This, coupled with what Davis found in Flathead Valley in the way of older, more extended moraine, seems to show that the great trough of the Flathead-Clark Fork from Dixon down to Thompson Falls has had ice in several places if not throughout its range. This, of course, was not the Wisconsin ice.

In January 1935, Bretz wrote Richard Foster Flint at Yale. The two had been corresponding for years, sharing observations and exchanging questions,

Exceedingly gravelly glacial deposits are also encountered in the Polson moraine south of Flathead Lake and in the moraines near Malaga [Washington] a few miles down the river from Wenatchee and in these two places the morainic topography is simply unquestionable, though the sections never would make you think of "till".

Lower Flathead River. The Flathead River below SKQ/Kerr Dam is deeply incised into glacial sediments and some bedrock. Prior to dam construction, the unregulated level of Flathead Lake stood at ~2880 feet elevation. My photo.

The USGS entered the region during, but more seriously after WWII. Joseph T. Pardee, having completed a new geologic map of the Colville Indian Reservation and a smaller study on the Latah Fm at Spokane, began publishing his findings on Glacial Lake Missoula deposits and drainage history. Rumor is he sat on the work for a few years due to office politics. Pardee's work on giant current ripples at Camas Prairie and on various other aspects of the lake basin have stood the test of time (Pardee 1910, 1940, 1942, Lee 2009). Pardee (1950) summarized glaciation of Mission Valley this way,

A Quaternary ice stream that originated in British Columbia moved south along the Rocky Mountain Trench and down its western arm into Mission Valley...The glacier left extensive sheets of drift which, as exposed in Mission Valley, indicate two advances of the ice.

William C. Alden (1953) and Gerry Richmond et al. (1965), informed by their own observations and the work of others that came before, delineated two terminal moraines in Mission Valley - one at Polson (Pinedale age) and one near St. Ignatius (Bull Lake age). Richmond and Roald Fryxell (1965, Part E, Stop E-23, Fig. 25) describe three stacked tills separated by three lakebed intervals in bluffs of the Flathead River at Polson.


Late Wisconsin Glaciation (MIS 2) - Most recent glacial advance in western North America. Roughly 35,000-11,700 years ago. Also called the 'Pinedale' Stage (Rocky Mts), 'Vashon' (Puget Sound), 'Fraser' (BC), and 'last glacial'.

Bull Lake Glaciation (MIS 6) - The older of two broadly recognized glacial advances in the Rocky Mountains and elsewhere. Applies mostly to the Laurentide Ice Sheet, except for the easternmost portion of Cordilleran Ice Sheet in NW Montana. Age range varies, but something like 150,000 years ago seems reasonable. Approximates the 'Illinoisan' Stage recognized in the Great Plains. 'Double Bluff' is likely the Bull Lake in Puget Sound. Bretz's 'Spokane' glaciation might be Eastern Washington's Bull Lake and likely correlative with 'early Wisconsin' till in Glacier National Park. Named for Bull Lake, WY.


The desire to expand Montana's hydropower production justified a fully-supported examination of the Lower Flathead River for potential dam sites (Soward 1965, Rice 1952). The geology at eight sites between SKQ/Kerr Dam and Paradise, MT (RM 3 to RM 72) was evaluated by Kenneth S. Soward, a few other USGS/USBOR geologists, and drillers. Detailed field descriptions of Quaternary deposits were collected and several test holes were logged at each site. Glacial till well south of the Polson moraine was described in numerous surface exposures and in drilling logs at 7 of 8 sites. Till predictably occurs in the same stratigraphic position, beneath lakebeds, at all seven sites. Geologic descriptions in Soward are lengthy, excellent, and little improved upon by later authors.

Dams never built. Red boxes are potential damsites on the Lower Flathead River identified by USGS (Soward 1965, Fig. 1). Glacial till was found at Perma, Damsite 4, Oxbow, Sloan Bridge, Mile 42.9 Damsite, Buffalo Damsite 1, and Buffalo Damsite 2. Yellow lines are moraines identified by Soward: 'Polson moraine' south of Flathead Lake, 'Kerr moraine' crossing the Valley View Hills, and 'Mission moraine' south of Charlo.

A pamphlet on the Channeled Scablands written for a general audience by Weis and Newman (1974) places the Flathead Lobe terminus at Polson.

Bedrock mapping by J.E. Harrison et al. (1986) gives no indication of a moraine south of Polson, but his focus was not on Quaternary deposits. No one is completely sure what his focus was on.

D.G. Smith (1966) mapped and described glacial features north of Polson, in Big Arm, a western embayment of Flathead Lake that contains terminal moraines of the Flathead Lobe. The lakeside village of Dayton is nearby. Evidence for two glacial episodes there includes,

...two levels of lateral moraine and two terminal moraines...the more recent of the two sets of moraines has a much greater volume of material, is fresher in appearance, and is more consistent in its morainal form

Remnants of the upper, older lateral moraine were found on the above Dayton at 4540', 4700', 4280', 5200' elevations, well above the prominent Late Wisconsin kame terrace. Using information from Big Arm area, Smith corroborates an earlier ice surface slope estimate of 50 feet/mile calculated by Alden (1953).

Two moraines. D.G. Smith (1996, Fig. 9) identified older, Mission moraine-age tills beyond the limits of Late Wisconsin moraines on hillslopes above Big Arm west of Flathead Lake.

Melville R. Mudge (1976) mapped a pre-Pinedale till ('foothill facies') in Swan Valley, the valley immediately east of the Mission Mountains.

Irving J. Witkind (1976) also reported finding two tills in Swan Valley near Yew Creek as well as kilometer-scale glacial grooves at the north end of the Mission Mountains near Bigfork (Witkind 1978).

Weber (1972) identified three different drifts deposited by alpine glaciers in the Bitterroot Mountains, the range due south of the Missions.

In Glacier National Park, Pinedale till (~20 ka) and Bull Lake till (~150 ka) are well documented (Horberg 1954, Richmond, 1957, Karlstrom 1981, Cioppa et al. 1995). Pinedale, Bull Lake, pre-Bull Lake, and even Pliocene tills are documented on the plains east of the park (Fullerton et al. 2004). Hauptman & Todd point out a consistent relationship between the terminal positions of moraines east of GNP,

Northern Montana was invaded by the Keewatin ice sheet [Laurentide Ice Sheet] at the Illinoian or the Iowan stage...Later advances [Bull lake and Pinedale]...terminated progressively farther north...

Consultants Peterson and Konizeski (Walker & Associates, 1974) prepared a fairly generalized geologic map of the Valley, locating the Mission moraine just north of Post Creek.

Way down here. Breckenridge (1989, Fig. 2) places the terminus of the Cordilleran Ice Sheet (Flathead Lobe) well south of Flathead Lake, at the south end of Mission Valley.

Briar patch. A satisfyingly scruffy map from a University of Montana graduate student showing both alpine moraines descending from the Missions Mountains and ice sheet moraines on the floor of Mission Valley. Ice sheet sub-lobe termini in Big Draw and in Little Bitterroot Valley also shown (Briar 1987, Fig. 4-13).

Keith L. Stoffel (1980) closely examined outcrops in Mission Valley and along the Flathead River. His detailed stratigraphic work, like that by Soward and Levish, is the best available. He interpreted tills this way:

'Jocko Valley diamicton' = Pre-late Wisconsin w/ terminal moraine near Arlee and Dixon

'St.Ignatius' till = Early Wisconsin (?) w/ terminal moraine near Ravalli Hill

'Early Mission' till = Middle to Late Wisconsin (even earlier Pinedale) w/ terminal moraine near St.Ignatius

'Late Mission' till = Middle to Late Wisconsin (early Pinedale) w/ terminal moraine near St.Ignatius

'Polson' moraine = Late Wisconsin (Pinedale) w/ terminal moraine at Polson

Volcanic ash in Mission moraine. I recently discovered this pristine airfall tephra in a sandy, stratified deposits (fluvial terrace) inset into older diamict (till) along Lower Crow Creek. Possibly Glacier Peak G (11,200 years BP)? Stoffel did not report this ash in his Crow Creek Sections 1 and 2, located up and downstream of this site (Stoffel 1980, p. 80-81). Outcrop is located on the north side of the creek 1220m southwest of the roadway crossing over Crow Dam's new spillway and 300m due west of the MA Canal headworks (47.4975, -114.2443, ~2790 ft elevation). Ash sample LCCN-1. March 2024 photo.

Waitt and Thorson (1983, Fig. 3-1) and Waitt et al. (2009, Fig 5) show the Flathead Lobe terminating at Post Creek.

Steve Porter (1983) and later Ken Pierce (2003, Fig. 1) placed the terminus at Polson.

A map in D.W. Briar's Master's thesis (Briar 1987, Fig. 4.13) shows the Mission moraine terminating at the National Bison Range. Briar's committee included Professors William Woessner, Johnnie Moore, and Jack Donahue.

Widespread till. A general geologic map by a USGS hydrogeologist (Slagle 1992, Fig. 2) depicts glacial till throughout Mission Valley, including Irvine Flats (ice sheet), the Dixon Bench (ice sheet), and areas between Ravalli and Evaro (alpine glaciers). Slagle's map is compiled from Ross et al. (1955) and Boettcher (1982).

Steven E. Slagle (1988) found no evidence for glacial ice at Irvine Flats, west of Polson, but provides a clear description of the Mission moraine,

The Mission moraine, interpreted by Alden (1953) as a series of imperfectly developed recessional moraines, occupies a crescent-shaped area in the southern part of Mission Valley. This moraine extends from the foot of the Mission Mountains on the east to the Flathead River and the hills on the west, and is generally bounded on the north by Crow and Spring Creeks and on the south by Mission and Post Creeks. The surface of the moraine is characterized by a swell-and swale topography containing numerous undrained depressions. The extreme southern part of the moraine is covered by remnants of lacustrine silt deposited by glacial Lake Missoula. Examination of well logs indicates that wells drilled into the Mission moraine typically penetrate a sequence of clay containing gravel, cobbles, and boulders (glacial till) and commonly terminate in gravel or sand and gravel. Surface exposures indicate that these sands and gravels represent intercalated layers within the till. At some locations no sand or gravel horizons are present and wells near the margins of the valley are drilled into bedrock to obtain water...The rest of the Mission Valley, not including Irvine Flats, is mantled by ground moraine deposited by various advances of the Flathead glacier. Alden (1953) believes that the ground moraine also underlies part, if not all, of the previously discussed terminal and recessional moraines.

In two maps of the Cordilleran Ice Sheet margin drawn at two different scales, Roy M. Breckenridge depicts the terminus of the Flathead Lobe well south of Polson at Post Creek (Breckenridge 1989, map on Table of Contents page and map in Chapter 3, Fig. 2)

In 1990, the U.S. Bureau of Reclamation prepared a seismotectonic assessment for four federally-owned irrigation dams operated by the Flathead Indian Irrigation Project (Ostenaa et al. 1990). Nine of the dams are located within 20km of the Mission Fault, the bounding structure that forms the steep west flank of the Mission Mountains. The team mapped Quaternary normal faults and glacial deposits throughout the Mission and Camas Valleys. Undeformed sediments were observed to overlie the Mission Fault at several locations, pushing the estimated age of last movement back to about 15,000-20,000 years ago. They confirmed earlier findings by L.H. Noble (1952) for the Mission Fault and C.E. Erdmann (1944) for the Flathead Fault.

Ostenaa et al. identified two terminal moraines of the Flathead Lobe in Mission Valley and three generations of alpine moraines along the Mission Mountains front. They argued against the presence of an ice shelf,

...we infer that the diamicts that comprise the Mission moraine were deposited directly under or next to glacial ice, not in a glacial lake from iceberg rafting or by "rain-out" of suspended lake sediment. Its morphology, lithology, and stratigraphic setting suggest that it formed subaerially...

The Ostenaa team mapped tills this way,

Polson till (tp) - Polson moraine, Latest Wisconsin

Kicking Horse till (tkh) - Mission moraine, Late Wisconsin

Ninepipe till (tn) - Mission moraine, Mid-Wisconsin

Dublin Gulch till (tdg) - Mission moraine, Early Wisconsin

St.Ignatius till (tsi), Pre-late Wisconsin (>130 ka)

Four potential tephras sampled by the Ostenaa team and submitted for analysis were determined by Franklin Foit at Washington State University not to be ash (Appendix B),

...volcanic ash layers within varved silt and clay of Lake Missoula overlying Dublin Gulch till [Mission moraine]...none of the sampled layers...contain a chemical fingerprint characteristic of volcanic ash...

A map by Paul Carrara (1995, Fig. 1) places the southern limit of Flathead Lobe at Charlo, MT.

A regional map showing the maximum extent of the Cordilleran Ice Sheet by D.B. Booth et al. (2003, Fig. 18) places the terminus of the Flathead Lobe at the Mission moraine.

Over the years, various university faculty and their graduate students have filled in details. Dan Levish (1997) sorted kettles from pingos and greatly refined our understanding of glacial sedimentation and the nature of the moraine-like ridges in Mission Valley. Master's students Rich Chambers (Chambers 1971) and Michelle Hanson (Hanson 2013, Hanson and Clague 2016) made important new discoveries in Glacial Lake Missoula varve sets. Professor Larry Smith, now retired from Montana Tech, has in a series of papers synthesized a mountain of information on flood deposits in the Clark Fork drainage. His work has caused many to rethink the nature of GLM and the timing of scabland floods leaving Montana (Smith, 2000, 2004, 2006, 2009, 2014, 2021, Smith et al. 2023). Hoffman & Hendrix's EdMap of the St. Ignatius-Arlee area identifies alpine tills of Bull Lake-age, but no evidence of the Flathead Lobe that far south. Other EdMaps completed for nearby quadrangles leave much to be desired. Long time geology Professor Rob Thomas (University of Montana Western in Dillon), who authored the newly-revised Roadside Geology of Montana (Hydman & Thomas 2020), locates a Bull Lake-age Mission moraine near St. Ignatius along with an outwash plain south of Post Creek. Graduate theses (Bondurant 2005, Braden 2006, Edward 2006, Salmon 2006) document striated bedrock south of Polson moraine. Larry Smith's 2004 field guide notes,

...a series of topographic ridges that appear to fan to the southwest...made up of diamicton

Groundwater reports by Montana Bureau of Mines and Geology are curiously reticent to identify glacial till south of Polson. A cross section through Pablo Reservoir (LaFave et al. 2004, Fig. 10b) shows no till in the subsurface, which contradicts maps and text found elsewhere in the same report (i.e., Figs. 8 and 9),

...some diamictons (till) in Polson moraine and Valley View Hills were deposited by glacial ice; some till may also occur in subsurface in the Mission valley

Baker et al. (2016, Fig. 7) places the Flathead Lobe terminus well south of Polson, while contemporary USGS publications locate it at Polson (Balbas et al. 2017, O'Connor et al. 2020, Waitt et al. 2021).

Bison Range. Fenced-in buffalo graze the open slopes above Moiese, silhouetted against the high Missions.

Mission Valley has two end moraines. Evidence for two major pauses is preserved on the floor of the valley. Glacial ice reached the southern end of Mission Valley and deposited the Mission moraine. Morainal ridges (Blue) are found as far as 40 km south of the Polson moraine (Red). My cross section above corresponds with Ostenaa's mapping. Topography from Google Earth. Whether the Mission and Polson moraines are from the same stage ('early' and 'late' MIS 2) or from two different stages (Bull Lake and Pinedale) is not known. What is clear is a regional pattern - older terminal moraines south, younger terminal moraines north - found elsewhere along the Cordilleran Ice Sheet margin (i.e., Bitterroot Valley, Puget Sound, Eastern Washington) and that of the Laurentide Ice Sheet in the Midwest and New England. In Puget Sound, for example, the Double Bluff terminus (MIS 6) lies south of the Fraser/Vashon limit (MIS 2) with Possession-age deposits (MIS 4) sandwiched between (Troost 2016).

Craggy face. Mt. Calowahcan (formerly named Mt. Harding) is one of many airy summits in the Missions, one of the least-visited ranges in the West. My photo.


a.) Is the Mission moraine real?

In Geology, there is a strong tendency to reference the work of your superiors and well-known authorities (i.e., textbook authors). Doing so is necessary and proper, but can sometimes be taken as an endorsement of others' observations and interpretations. Graduate students almost never disagree with their advisors. New hires at USGS rarely contradict their bosses lest they face transfer to Kansas. And so on.

The literature on the Mission moraine is a pertinent example of this sort of perpetual citation without investigation. There are two camps, one for its existence and one against. Articles by Noble, Alden, Davis, Richmond, Soward, Decker, Peterson & Konizeski, Stoffel, Curry, Ostenaa, and Hydman & Thomas all recognize the Mission moraine and place it between Ronan and St. Ignatius.

In agreement with previous research in the valley...we infer that these units were deposited directly under or next to glacial ice under subaerial conditions, not in a glacial lake from iceberg rafting or "rain-out" of suspended lake sediment, although sedimentologic and stratigraphic studies are necessary to confirm an origin as till.

Articles by Levish, Smith, Ryan, Hoffman, Hendrix, and several University of Montana graduate students are skeptical the Flathead Lobe ever advanced south of Polson.

Ostenaa's map of multiple nested moraines is clear and convincing, yet Levish successfully demonstrates those tills are atypical and not deposited by grounded ice. They are subtly stratified and partly water-laid - deposits rained from a floating ice shelf. The ridges are not particularly bouldery when viewed from a distance, but closer inspection reveals many embedded boulders. In short, the landforms on the floor of Mission Valley often disagree with the deposits. Our mental picture of what a terminal moraine should be - the images in textbook - doesn't work very well for Mission Valley. The standard sedimentation models and facies associations fail us, as do terms like Bull Lake and Pinedale.

Ostenaa et al. (1990) list a number of locations in the greater Glacial Lake Missoula basin that hint at the presence of a pre-MIS 2 Glacial Lake Missoula deposits and deserve another look:

a.) Ninemile Section. Lakebeds in a high terrace above the type section of Chambers (1971) may be pre-MIS 2.

b.) Paleosols (argillic and calcic horizons) at Dublin Gulch and near Post Creek may be pre-Late Wisconsin.

c.) Lakebeds of an early GLM overlie till of the Flathead Lobe at Dublin Gulch, Post Creek, and Lower Crow Reservoir.

d.) Deltaic deposits at Dry Creek east of St. Ignatius record several periods of lakebed and outwash accumulation.

e.) Lakebeds interfinger with tills southwest of Ovando, MT.

f.) Lakebeds overlie coarse outwash(?) north of Greenough, MT.

g.) Late Wisconsin outwash buries lakebeds north of Salmon Lake, MT.

h.) Lakebeds overlies drift of uncertain age north of Cottonwood, MT.

i.) Varved silts interfinger with till from a glacier that occupied the Clearwater Valley, MT.

j.) Deltaic sediments that appear to be subaqueous underflow deposits in a glacial lake east of Clearwater Valley, MT.

b.) Polson moraine morphology

Many have noticed the complexity of the Polson moraine. Constructional morainal crests and sets of subridges that parallel one another, forming coherent swaths of till are mappable in the lidar image. Ridge crests show crosscutting relationships; at least 3 distinct subunits (minor advances?) can be distinguished. Ice lingered and shifted at Polson.

c.) Similar number of alpine and ice sheet moraines

Compound alpine moraines along the Mission front attest to at least 3 generations of mountain glacier growth and ice advance. A similar number of advances is preserved in moraines deposited on the valley floor by the Flathead Lobe.

d.) A conspicuous shift in the literature

Little attention today is paid to pre-late Wisconsin deposits of the Cordilleran Ice Sheet. Its a glaring gap that seems to have developed in the literature in the 1960s-70s in large part due USGS's influential summary article (Richmond et al. 1965), which reinterpreted Bretz's pre-Late Wisconsin glacial margins near Spokane as late Wisconsin. But Eastern Washington's glacial geology has never fit Richmond's Rocky Mountain framework. Speculative till at Spokane and cryptic photos of other pre-Pinedale deposits pepper articles between 1900-1950, but none determines their absolute ages. Booth et al. (2003) succinctly summarizes the two-pronged problem,

Discontinuous drift extending beyond the limits of Fraser-age drift in the Pend Oreille, Columbia, and Little Spokane valleys has stones that are highly weathered or deeply penetrated by cracks, has a slightly argillic soil, and overlies granite and gneiss bedrock that is highly decayed, even to grus...Direct dating of pre-Fraser sediments is poor...In northeastern Washington and adjacent Idaho, however, there is no objective basis for Richmond's (1986, Chart 1) assignment of any of these deposits to particular time intervals.

The shift away from pre-late Wisconsin to late Wisconsin seems to correspond with the arrival of an ambitious crop of Baby Boomer geologists. Ideas forwarded by the bright new hires at USGS and various universities were accepted without friction by the larger geoscience community. Admittedly, it is difficult to point to any one article that caused the community to turn away from the older glacial record, but several factors seem to have collectively taken a toll: Improved absolute dating techniques (so promising, so fashionable!), the frustratingly subtle expression of pre-Pinedale deposits along the CIS margin (not worth the time), turf-protection by flood geologists (same people for 50 years), and a general shift away from field work (its hot out here). The shift occurred after Richmond et al. (1965) and Baker (1973) - somewhere within the first decade of scabland articles published in the post-Bretz era.

Charting a change. Starting in the 1970s, discussion of pre-late Wisconsin ice and older glacial deposits begins to fade from publications about the Rocky Mountains and Pacific Northwest. I put this chart together to see if I could tease out a pattern. It still doesn't make sense from a science point of view. I suspect people lost interest, specifically geological survey staff and journal editors.

e.) Boulders on the valley floor

Boulders strewn about the floor of Mission Valley were deposited in two ways. Some are dropstones floated in on ice bergs. These are typically large, singular boulders either resting proud of the surface or slightly embedded into it. Others occur in piles made by man. These 'farm piles' were created when land was first cleared for agriculture. Both contain information. Ice-rafted erratics are more or less randomly distributed within the valley below the highest shoreline of Glacial Lake Missoula. Farm piles occur in higher densities in certain places, especially on low ridges.

New erratic mapping. I've begun mapping the locations of ice-rafted erratics and 'farm piles' in the Mission Valley. Early results indicate both very large boulders (orange) and farm piles of smaller boulders (blue) tend to cluster at the base of alpine moraines along the mountain front and on low ridges crossing the valley floor mapped as moraines by Ostenaa. I've placed >600 points so far, but the dataset is far from complete.

f.) Pingos on flats uphill of moraines

Pingos are ice-cored hills formed by upwelling groundwater in periglacial settings. Clusters of pingos, many of which are now hollows filled with water, pepper the floor of Mission Valley. Pingo-pond clusters are dense north of the horseshoe ridges, but sparse south of them. Their distribution is consistent with stepped retreat of grounded ice (retreat-pause-retreat). Pingo ponds are found in abundance along Kruse Lane, Johnson Road, Mollman Pass Road, and Duck Road. Pingos are absent from alpine moraines, the drift train along the Mission front, gravel bars along Flathead and Jocko Rivers, and the Polson moraine.

g.) Kettles on ridge crests and slopes downhill of ridges

Kettle ponds occur with the pingos. They tend to have less circular shapes in map view. Kettles form when irregular blocks of glacial ice get buried in outwash, melt, and the ground around them collapses inward. They require grounding of bergs (i.e., no glacier or lake on valley floor). Kettle-ponds lack the elevated rims of pingos. They form less dense clusters atop the horseshoe ridges and are loosely distributed across low-angle slopes south of them (i.e., ice bergs stranded on former shorelines). Some correspond with small landslides, while others appear to form the heads of small channels (i.e., colluvial hollows). Kettle ponds are also found on the Polson moraine (i.e., Turtle Lake). Very few are found on alpine moraines and the drift train along the Mission front. Several occur on gravel bars along Flathead and Jocko Rivers as well as the Polson moraine.

h.) Lack of exposed Tertiary rocks in Mission Valley

Contrast the Mission Valley with the Bitterroot Valley, its neighbor to the south. Tertiary strata should onlap Precambrian bedrock here like it does there. Tertiary rocks should crop out all along the flanks of the Valley View Hills, Moiese Hills, and the Bison Range, but do not. The Tertiary here only occurs in deep wells beneath a thick pile of glacial sediment. Wells drilled near the south end of the valley penetrate >300m of mostly Quaternary fill before reaching bedrock (A.J. Boettcher 1982). Tertiary strata comprise the valley's deep aquifer (Slagle 1988, Smith 2004). So, Tertiary basin fill sediments were deposited here, just deeper. Mission Valley in pre-glacial times was certainly a much deeper trough than it is today.

Ice that advanced into Mission Valley during Bull Lake time or earlier, would have encountered a largely unfilled Mission Valley. The top of an early Flathead Lobe could easily have been 100-200m lower than that of the Pinedale ice sheet. Glacial ice invading a deeper valley might never have come close to overtopping the Valley View Hills. Levish could be entirely correct: Bull Lake ice never overtopped Valley View Hills and the Pindale ice sheet never moved south of Polson.

Did ice scour basin fill sediments from Mission Valley? Ostenaa et al. (1990) entertained the idea, one that requires the scouring occurred during Bull Lake or older stages,

The Mission Valley is one of the few, perhaps the only, normal fault-bounded valleys in the western cordillera in which a thick section of Tertiary deposits is not well exposed...All of the surficial deposits mapped in the Mission Valley by this study and by previous workers are of Quaternary age. All of the areas where Tertiary units have been identified in drill holes lie outside the limits of most of the major advances of the Flathead lobe, and lie outside the range of low hills (Valley View Hills)...The lack of Tertiary outcrops may be due either to the extensive history of Quaternary glaciation in the Mission Valley or to a lack of Tertiary deposition.

Older bigger, younger smaller? Ice during older glaciations (blue) appears to have advanced as far or farther south than ice from the last glacial (red). Older end moraines peek out from beneath youngest ones.

i.) One or two Glacial Lakes Missoula?

Geologists disagree about the number of times Glacial Lake Missoula filled and spilled. Ambiguity surrounding an older GLM is noted by several Montana geologists, including Johns (1970),

Although a major glacial lake may have been impounded in the same place earlier, possibly several times, only the last one is considered here.

Did GLM fill during Bull Lake and Pinedale time? OSL, tephra, and radiocarbon dates from various locations bracket the lake between 20-14 ka (Konizeski et al. 1968, Levish 1997, Kuehn et al. 2009, Hoffman and Hendricks 2010, Hanson et al. 2012, Smith 2004, Smith et al. 2018, O'Connor et al. 2020). The lake seems to have dried up by about 13.5 ka. Pre-MIS 2 fillings and spillings seems to be mostly ignored, though good evidence for two tills in the Flathead and Mission Valleys (Soward 1965, Stoffel 1980) and boulder gravels left by a few pre-Wisconsin floods in the Channeled Scablands exists (Baker 1973, Foley 1982, Baker et al. 1991, Bjornstad et al. 2001, Cooley 2023).

j.) Does floating ice create horseshoe-shaped moraines?

How would sediment dropped from a floating ice tongue form horseshoe-shaped ridges that so closely resemble moraines left by grounded ice? The sedimentary process whereby diamict rained through the water column piled up on the lakebed is unclear. Of the five facies associations for tidewater glaciers illustrated in Powell (1981), only one shows cross valley ridges. Ridges in Powell's Figure 1 appear to be either 'push moraines', which are ice-contact landforms (Evans and Twigg 2002, Sharp 1984), or deposits of supraglacial streams,

If substantial supraglacial debris is dumped off the ice as it calves, it will remain as piles of rubble and gravel on the fjord floor.

Evans (2013) adds some detail to 'push moraine',

Push moraines are broadly arcuate in planform, but in detail are often irregular and winding, reflecting the morphology of the glacier snout...Where glacier snouts are quasistable...large terminal moraines may accumulate by annual accretion.

Typical terrestrial glaciers form moraines during pauses in advance or retreat. Contiguous, horseshoe-shaped moraines are common because they follow the shape of the glacier's snout. Internal flow and drag against bedrock along its margins conspire to make round noses. Would a floating ice tongue in a 15-mile wide fjord have a horseshoe-shaped snout? Would that shape hold as ice retreated, leaving behind multiple concentric ridges? The ice tongue would have been an intact sheet, not a flotilla of large bergs. I would expect random piles and longitudinal ridges to form beneath a tidewater glacier, not contiguous cross-valley horseshoes.

Ambiguous ridges. The first of five facies associations for tidewater glaciers is shown in this sketch by R.W. Tope in Powell (1981, Fig. 1). A discontinuous gravel ridge is described as either a 'push moraine' or a deposit spilled off the front of the glacier by a supraglacial stream during a stillstand. What explains its cross-valley orientation? Is this a horseshoe?

k.) Survey of tidewater snouts

According to the classic textbook of 'Glacial and Quaternary Geology' (Flint 1971),

The termini of glaciers that end in water and calve icebergs are irregular or nearly straight, and many are concave downstream.

I used Google Earth to peruse the snouts of ~100 tidewater glaciers in Alaska, Svalbard, Greenland, and Russia. Only images from 2020 or later were viewed. Most tidewater margins were quite straight, gently arcuate, and sometimes jagged. Horseshoe-shapes (convex downstream) were rare. In wide fjords without rock islands at their mouths, the shapes of ice fronts seem directly linked to the frequency of calving than anything else (lots of bergy bits in photos). Crevasse orientations, snout position within the fjord, and other things also affect shape. A few typical examples are shown below.

Glacier Bay National Park. The snouts of three tidewater glaciers in Alaska - Reid, Hopkins, Grand Pacific - are fairly straight, shapes similar to those seen today. National Park Service photo c.1900.

Svalbard. Snout of Kronenbreen glacier is fairly straight. Google Earth image.

Prince William Sound. Gently concave snout of a recently-calved tidewater glacier in south-central Alaska. Google Earth image.

Svalbard. Snout of Dahlbreen glacier is irregular with a few convex (horseshoe-shaped) portions. Google Earth image.

Greenland. Sublobe forms shaped by recent calving of a glacier east of Murcheson Channel. Google Earth image.

Russia. Two concave, one convex. Is that a horseshoe? Severny Island, Russia. Google Earth image.

Snout shape. Red and blue lines trace the shape of an ice front in retreat over a 62-year period. Red = Convex, Blue = Concave. Snout was convex when the ice was at the mouth of the fjord, fairly straight where the valley narrows, and concave at retreated positions.

l.) Jette Hill vs. Post Creek Hill

The size of these two hills, both composed of till, is similar. Arguments that Post Creek hill is not a moraine, but formed by faulting or fluvial incision are not well supported. Jette Hill is mostly a moraine.

More to come on this...

m.) Post-GLM incision of Flathead River

Deep incision of the Lower Flathead River and a distinct pattern of bluff retreat occurs between SKQ/Kerr Dam and the mouth of Crow Creek. The incision appears to have been rapid, likely caused by the cutting of a new bedrock gorge at the damsite. Downstream of Crow Creek the river widens abruptly and bluff gullying stops. The river lies >50m below the broad flats above breezy Buffalo Bridge, entrenched in lakebeds and diamict. Incision appears to have happened late, likely after the Flathead Lobe had retreated from the Polson moraine (post-LGM; Timmerman 2005, Figure 11, Time 2). Did the river previously follow a route out of the lake, along the east side of Valley View Hills, swing SE passing west of Mission Valley Golf Course, and enter the Crow Creek drainage at the east end of where Lower Crow Reservoir is today?

n.) My current thinking

The so-called Mission moraines are either Late Wisconsin (MIS 2) or 'Bull Lake' (pre-MIS 2). If Bull Lake and composed of material rained from the base of a floating ice shelf through a column of water, then there were two Glacial Lake Missoulas, one at MIS 2 and one prior to MIS 2. Large ice dams to the west would have formed during two different glacial stages. If the moraines are Pinedale age and products of deposition from floating ice, then the Polson moraine is not the terminal position of the Flathead Lobe, as is stated in many articles published over the past century.

In one camp are Larry Smith, Marc Hendrix, and Dan Levish. All cast doubt on the existence of a Mission moraine, interpret sediments that comprise the ridges in Mission Valley as glaciolacustrine, or place them in MIS 2. In the other camp are Chamberlain, Nobles, Alden, Davis, Bretz, Richmond, Soward, Decker, Peterson & Konizeski, Stoffel, D.G. Smith, Breckenridge, Carrara, Chambers, Curry, Waitt & Thorson, Ostenaa, and Thomas. All observed a set of moraine-like ridges well south of Polson and depict them on maps.

I don't yet know where Jim Sears stands. Rumor has it he has mapped much of the Mission Valley, but has not published.

To my eye, the Mission moraines - all of the low ridges south of Polson - are recessional features; Ostenaa's mapping is correct. The ridges are less bouldery and sedimentologically odd, yet nearly identical to those that comprise the Polson moraine, which nearly everyone argues was deposited in the usual way, by ice moving over ground. The Mission moraines were always smaller than the Polson with topographic profiles subdued over time by erosion. Each represents a single, short pause during ice termination and retreat. The southernmost terminus at St. Ignatius was never a strong ridge and is now all but gone. Diamict, which cores the ridges, is nearly everywhere overlain by lakebeds. The lack of scour high on the Valley View Hills, as Levish argues, could be explained by a deeper valley floor during Bull Lake time and a flat ice profile (i.e., modern day Matanuska glacier).

With respect to the Mission moraines, the landforms and the sediments disagree. That doesn't bother me. Moraines in NW Montana (and in Puget Sound) don't look like they do in the Alps (or textbooks). I prefer a 'late' Bull Lake or 'early' Pinedale age and deposition by glacial ice on the valley floor. A floating ice shelf would not deposit contiguous, nested, horseshoe-shaped moraines. Convex morainal landforms are the products of grounded ice.


Evidence for two terminal positions of the Flathead Lobe are preserved in Mission Valley, MT. New mapping on a lidar base confirms a set of nested ridges composed of diamict well south of the Polson moraine. The ridges in Mission Valley cross the valley floor and span gaps between sculpted bedrock hills. They are horseshoe-shaped recessional features deposited by the retreating Flathead Lobe prior to formation of the Polson moraine. If the ridges are products of either a 'late' Bull Lake or 'early' Pinedale glaciation, but not products of grounded ice, then the Flathead Lobe floated on an older Glacial Lake Missoula (MIS 4 or 6). If the ridges were formed during an early phase of the same advance as the Polson moraine, then both the Mission and Polson moraines are MIS 2 ('early' and 'late' Late Wisconsin, respectively). Regardless of whether the Mission moraines are MIS 2 ('early' Pinedale), MIS 4, or MIS 6 ('late' Bull Lake), the Polson moraine is not the southernmost terminus of the Flathead Lobe.

Three options. The moraines in Mission Valley belong to MIS 2, MIS 4, or MIS 6. Option A envisions two glaciations (Bull Lake, Pinedale) and assigns the Mission moraines to Bull Lake/MIS 6. Option B assigns the Mission moraines to MIS 4, a stage not well recognized in the region. Option C requires no Bull Lake and instead divides MIS 2 into two parts, 'early' and 'late'. The Mission formed during 'early' MIS 2, the Polson during 'late' MIS 2. Option C is most consistent with OSL-dated deposits in the Clark Fork drainage (Smith 2023). Regardless of option, the Polson is not the terminal moraine of the Flathead Lobe, the Mission is. The Polson represents a prolonged recessional pause during the final northward retreat of Cordilleran ice from Montana. Figure redrawn from a sketch by Nick Zentner of CWU (YouTube 1 Feb 2024). 'Smaller' and 'Bigger' refer to the relative size, erosional work done by, and southerly extent of glaciations.


Early hints of two moraines. Red box defines the Mission Valley. Mapping by the USGS geologist Gerald Richmond (Richmond et al. 1965) identifies four moraines: "Be" (early Bull Lake), "Bl" (late Bull Lake), "Pe" (Wisconsin Polson moraine early), and "Pm" (Wisconsin Polson moraine middle). The pattern generally agrees with similar work done in the Bitterroot Valley, MT. There, deposits from 3 cycles of alpine glaciation were mapped (Alden 1953, Weber, 1972). Smith (1966) believed Big Draw, located north of Polson and west of Flathead Lake, contains evidence of a pre-Polson moraine (T24N, Rge 23W, Sect 16; middle Wisconsin?). Chambers (1971, Fig. 2) uses this map.

Younger north, older south. Two terminal moraines exist in Mission Valley, MT. The Polson moraine is a hummocky ridge that stands prominently above the valley floor, forming a natural dam to Flathead Lake. Richmond mapped this as it "Pe" and "Pm". It is by all measures a classic end moraine. The older Mission moraine is different. The Mission advance, located farther south, has no prominent end moraine. Rather, it consists of a set of sparsely-bouldered, low-relief, nested horseshoe-shaped ridges that span the valley between bedrock uplands. The ridges are weakly incised by dozens of small, now-dry gullies that head near their crests. Richmond mapped the southernmost ridge as "Be". Though the heights of the older Mission and younger Polson moraines differ, their widths and planform shapes are nearly identical. Map above modified from Levish (1997) using Nick Zentner's color scheme.

Mission Valley section. The Mission Valley is east of the Purcell Trench Lobe at the south end of the Rocky Mountain Trench (Daly 1912) between the cities of Missoula and Kalispell. Stratigraphic sections in glacial deposits (far right side of figure) are ostensibly below the Ninemile and Garden Gulch sections of Michelle Hanson and Larry Smith. Similar to the Wenatchee Valley in Washington State, the Mission Valley spans the former margin of the Cordilleran Ice Sheet, but lies 670km to the east and 600km upstream. Wenatchee, WA (200m elevation) and Ronan, MT (930m elevation) fall along the same latitude line. The lower Flathead River channel, which begins in Flathead Lake at Polson, is deeply entrenched and shoved hard against the valley's western margin. The river is joined by the Jocko River at Dixon and empties to the Clark Fork River at Paradise, ultimately finding the Pend Oreille and the Columbia Rivers farther downstream. Figure modified from O'Connor et al. (2020).

Striated bedrock. Glacial grooves and polished bedrock surfaces in Glacial National Park. USGS/M.R. Campbell photo, 1913.

The big question. Just how far south did the Flathead Lobe advanced into Mission Valley during older glacials? Some believe ice advanced no farther than the Polson moraine. Others suspect ice advanced all the way to St. Ignatius. The photo above and others below show outcrops of glacially-polished and striated bedrock located several kilometers south of the Polson moraine. The quartzites and argillites are part of the Precambrian Belt Supergroup. The striated surfaces were uncovered several years ago during construction of the adjacent highway (Back Road near Goose Hill). The sediment cover protected them from erosion and roughening, similar to the way striated surfaces on granite in upper Grand Coulee were protected by silt (Bjornstad in Waitt et al. 2021, Figure 7a). Rumors of striae along Back Road have been floating around for years, but evidence has remained unphotographed. I recall discussing this location with Mike Stickney of MBMG a few years ago. I visited in September 2023 and was surprised how clear the scratches were, preserved in a dozen or more small outcrops above the road. My photo.

Glacial ice reached Valley View Hills. Smooth, sculpted bedrock ledges lie beneath a veneer of loose cobbly, bouldery outwash and patchy clay-matrix till(?). The north-south grooves and flutes are independent of bedding and structure. Levish used a contour map of the Valley View Hills to argue ice never overtopped them. The hillslope profiles are more consistent with dissection by small streams than by glacial ice. Reasonable argument, but we don't know the profile of the Flathead Lobe and if ice occupied the valley at an earlier time, the valley floor might have been significantly lower (less outwash gravel, less lake bottom silt). Either way, if you hike the crest of the Valley View Hills (Goose Hill or the 'bare knowl' ascended by David Thompson, Lat 47.6435, Lon -114.1912), you sure get the impression ice made it well south of the Polson.

Glassy surfaces hide beneath glacial cover. Polished faces in the Valley View Hills west of Pablo Reservoir. The polish is only observable when the gravel cover is removed. F.A. Steere (1895) noted the same issue at Mt. Powell in Granite County, "By removing about two feet of debris and detritus from the sides of the gorge of Dempsey Creek...I found highly polished abraded surface well grooved". My photo.

Two scrape directions. In outcrops at road level, older horizontal striations - possibly made by glacial ice - are crosscut by much younger scrapes made by an excavator during construction of the highway. Lichen has grown in the decades since the highway was built. My photo.

Goose Hill. View north over Back Road and the tree-lined Polson moraine to Flathead Lake. The explorer David Thompson stood here more than 200 years ago, in 1812.

Glacial striae orientations. Braden (2006, Fig. 25) collected 186 measurements on glacial striae at 8 sites, seven of which are located north of the Polson moraine. Scour indicates ice movement in two orientations: NE-SW and E-W. Site JBOO105 is the Valley View Hills location along Back Road shown in photos below also visited by Middlebury College professor Peter Ryan (Ryan 1998, Smith et al. 2000, p. 45-47). Salmon (2006, Fig. 18) reported similar striae orientations at a site near JB03905. Bondurant (2005) reported NE-SW and E-W striae farther north in Big Draw and Elmo Valley. Johns (1964) reports striae with trends N40W to N45W near Lake Mary Ronan.

Previous moraine mapping. I broke out the colored pencils in order to highlight the morainal ridges on this wonderful map by Ostenaa et al. (1990). Ostenaa's team from U.S. Bureau of Reclamation improved upon earlier work by Elrod (1903), Davis (1920), DeYoung and Roberts (1929), Nobles (1952), Alden (1953), Richmond et al. (1965), Soward (1965), Peterson and Konizeski (1974), Curry et al. (1977), Stoffel (1980), Mudge et al. (1982), and Harrison et al. (1986). No moraines associated with the ice sheet were included on the 100k map of the Plains, MT Quadrangle (Lonn et al. 2007), an odd choice given the information available. Their mapping of alpine moraines along the east edge of the map sheet, likewise, leaves much to be desired. The pencil colors are arbitrary, chosen only for visual contrast (sorry Nick!). Pink is the most southerly glacial till they found (Mission moraine, Richmond's "Be" = early Bull Lake); the slopes west of St. Ignatius are conspicuously boulder-strewn. Yellow at the top of the map is the Polson moraine (Richmond's "Pm" = Wisconsin Polson moraine middle), which encloses Flathead Lake. Olive, Green, Orange, and Blue moraines (Richmond's "Bl" = late Bull Lake) are ridges formed during temporary still-stands or minor advances during the recession of pre-Wisconsin ice from its maximum position (Pink). Though the authors assign a different name to each till, the ridges do not each mark a separate advance. All four may be lumped into one advance. At least two, possibly three, generations of alpine ice spilled from the Mission Mountains front, seen along the right side of the map (Teal, Red, Yellow). Peach delineates coarse grained terrace gravels along the Flathead River deposited by Ice Age floods moving down the canyon. A curious swath of sand through the center of the valley occurs between the Polson moraine and the Flathead River-Little Bitterroot River confluence (Tan). The sandy swath, which has not attracted much attention from previous workers, may be a late spillover from Flathead Lake through a low gap in the moraine where it meets the mountain front at Bisson Creek. Light blue areas are post-glacial sand dunes. GLM lakebeds, Holocene alluvium, and bedrock areas were left uncolored.

Diamict at Polson. The Polson moraine divides the Flathead Valley to the north from the Mission Valley to the south. Diamicts, or matrix-supported conglomerate, that comprise the moraine are well exposed near the town of Polson along the road leading down to SKQ/Kerr Dam (completed in 1938). This is what morainal deposits in Mission Valley look like - what Levish interpreted to be sediments rained through the water column from the base of a floating shelf of ice onto the floor of a proglacial lake. Most later workers agree with Levish's narrative: the Flathead Lobe terminated as an ice shelf in Glacial Lake Missoula. Why would sediment dropped from a floating ice tongue form horseshoe-shaped ridges that so closely resemble moraines? The sedimentation process remains unclear. My photo 2023.

Diamict at Sloan's Bridge. Picturesque bluffs along the Flathead River at Sloan's Bridge 20km southwest of Polson. My photo 2021.

Diamict at Lower Crow Dam. Matrix-rich deposits mapped as till of the Mission moraine 20km south of Polson. My photo March 2024.

Red is till. East-west cross section across Mission Valley through Pablo Reservoir by Larry Smith (2004). Smith shows glacial till in two locations (red), 1.) At the range front of the Mission Mountains (e.g., till from alpine ice) and 2.) Along the east side of Valley View Hills (e.g., till from Flathead Lobe), where my photos of glacially-polished rock were taken. Smith hedges a bit by using 'Till / Alluvial Fans' in his unit description of the bouldery-gravelly deposit exposed along Back Road. 'Shallow alluvium' (blue, yellow, red) consists of all Pleistocene-age glacial deposits. 'Deep alluvium' (pale yellow) consists of sand and gravel beneath the till, pro-glacial lake bottom deposits, and outwash, but above Tertiary bedrock (gray).

Erosion works fast. Fins of late Wisconsin age diamict near Kerr Dam are constantly changing. Seasonal rains, snowmelt, and near constant sloughing conspire to degrade the unvegetated bluffs. Ridges composed of diamict such as this cannot persist for hundreds of thousands of years. The topographic expression of pre-Polson moraines should be diminished in comparison. My photo 2023.

Polson moraine badlands. Laminated silts (lakebeds) overlies morainal deposits (diamict), forming a band of badland cliffs near Kerr Dam west of Polson, MT. If you're into robbing banks, make sure to locate your hideout here. The northern Mission Mountains, which form the eastern margin of Mission Valley, are seen in the distance. The Polson moraine appears to have been breached by water escaping south out of Flathead Lake. One obvious spillover point was Kerr Dam. A second, less obvious spillover might be at Bisson Creek, against the mountain front. Flows moved southwest across valley towards Lower Crow Reservoir. USGS photo by Alden (1953).

How many Ice Age floods left Mission Valley? Map of flood gravels along Flathead River at Moiese, MT. In contrast to Chambers (1971), Waitt (1985), and Atwater (1986), Levish contends, "The stratigraphic record does not lend support to hypotheses that [Glacial Lake Missoula] drained catastrophically and completely many times". Soil mapping by DeYoung and Roberts (1929), Quaternary mapping by Ostenaa et al. (1990), terrace mapping by Edwards (2006), and bedrock mapping by Lonn et al. (2007) all describe high energy gravels in a huge, multi-terraced bar and large-scale anastomosing forms occur along the Flathead River near Moiese. Peterson and Konizeski (1974) imprecisely identify these substantial gravel bars as 'Qal'. In 2021, I mapped the surficial geology of the Sloan, MT 1:24k Quadrangle, identifying 3 terrace surfaces formed in boulder-cobble gravels and inset into varved beds of Glacial Lake Missoula. I interpret each surface as a separate draining of Lake Missoula, thus a record of 3 energetic floods leaving the Lake Missoula basin for the Clark Fork Valley. The gently downvalley-sloping terraces likely match with catastrophic flood terraces at Kerr Dam. Velocities were sufficient to transport boulders and very coarse gravel through a bedrock-confined canyon 2-3 km wide (Sloan Bridge to Bison Range entrance gate). Repeated, partial lake drainings (i.e, Baker and Bunker 1985, Smith 2004, 2006, 2021) is an interpretation supported by field evidence in Montana.

New surficial geology. My surficial geologic map of the 1:24,000 scale Sloan, MT Quadrangle (c. 2020) highlights 3-4 terrace levels along the Flathead River. The high energy gravels (Blue, Pink, Green) are inset into an older, varved lakebeds of Glacial Lake Missoula (Salmon). Levish et al. (1993 Table 2, sample OTL412) obtained an OSL date of 108 +/-11 ka on the lakebed silt. Risers separating gravel Terrace treads are several meters high. Channel patterns on terrace tops are truncated by the next younger terrace. Moiese Valley is west of the Moiese Hills and appears never to have been glaciated.