Two Terminal Moraines in Mission Valley, MT

"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." - Dan Levish

The statement above is a key finding of Levish's dissertation completed at University of Colorado Boulder under Dr. Peter Birkeland (Levish, 1997). I agree with Levish that classic hummocky, boulder-strewn moraines are largely absent from Mission Valley, but disagree that 'no record' exists for ice advance south of the Polson moraine. Older ice left an impression.

Way out east. The Mission Valley is located in northwestern Montana, where the Flathead Lobe of the Cordilleran Ice Sheet terminated. Map modified from Waitt (1980) and Levish (1997).

Two moraines. Remnants of two moraines are preserved in the Mission Valley, the Mission moraine (Blue) and the Polson moraine (Red). According to mapping by Ostenaa et al. (1990), the older Mission moraine (pre-Wisconsin? mid-Wisconsin?), lies 40 km south of the younger Polson moraine (late Wisconsin). This same pattern - older south, younger north - seems to hold for other lobes of the Cordilleran Ice Sheet from Puget Sound to Eastern Washington to Northern Idaho.

Two geologists. William C. Alden and Joseph T. Pardee with Fords in Montana in 1921. Both men did some of their best work in Montana. USGS photo.

Younger north, older south. Two terminal moraines in Mission Valley, MT (modified from Levish, 1997). The Polson moraine is a hummocky ridge that stands prominently above the valley floor, forming a natural dam to Flathead Lake. The Polson is by all measures a classic moraine. The older Mission moraine is different. The Mission is a set of low-relief, nested horseshoes that span the valley between bedrock ridges. The two moraines are nearly identical in their planform shape and scale.

Moraine mapping. Glacial geology between Flathead Lake and Jocko River. I broke out the colored pencils in order to highlight the morainal features on this wonderful map by Ostenaa et al. (1990) who improved upon earlier work by Elrod (1903), Davis (1920), DeYoung (1929), Nobles (1952), Alden (1953), Richmond et al., (1965), Peterson and Konizeski (1974), Curry et al. (1977), Stoffel (1980), Mudge et al. (1982), and Harrison et al. (1986). The colors are arbitrary, chosen only for visual contrast. Pink is the most southerly glacial till (Mission moraine), only partially preserved. Yellow at the top of the map is the Polson moraine. Green, Orange, and Blue moraines are likely temporary still-stand features formed during recession of pre-Wisconsin ice, not separate advances. At least three generations of alpine ice spilled from the Mission Mountains can be seen along the right side (Red, Yellow, Teal). Peach delineates coarse gravels along the Flathead River at left (Ice Age flood deposits) and a curious swath of sand through the center of the valley between the Polson moraine and the Flathead River-Little Bitterroot River confluence. The sandy swath, which has not attracted much attention from past workers, may be a spillover from late-glacial Flathead Lake through a low gap in its moraine near Bisson Creek. Light blue areas are post-glacial sand dunes. GLM lakebeds, Holocene alluvium, and bedrock areas were left uncolored.

Polson moraine. Morainal deposits near Kerr Dam west of Polson, MT with the northern Mission Mountains in the distance. USGS photo.

Moiese flood gravels. Levish contends, "The stratigraphic record does not lend support to hypotheses that [Glacial Lake Missoula] drained catastrophically and completely many times". However, soil mapping by DeYoung (1929), Quaternary mapping by Ostenaa et al. (1990), and bedrock mapping by Lonn et al. (2007) all note high energy alluvial deposits and anastomosing forms along the Flathead River near Moiese. Peterson and Konizeski (1974) imprecisely identify these gravels as 'Qal'. In 2021, I mapped the surficial geology of the Sloan 1:24k Quadrangle (Sloan Bridge to Bison Range), noting 3-4 terrace surfaces in the valley-confined boulder-cobble gravel train. I interpret each surface as a separate draining of Lake Missoula, so at least 3 floods out of the Lake Missoula basin, possibly more. The landforms and deposits in the vicinity of Moiese, MT are consistent with catastrophic flooding and partial lake drainings (i.e, Baker and Bunker, 1985; Smith, 2006). Velocities were sufficient to transport very coarse gravel through the 1.5 mile-wide bedrock canyon.

Surficial geology. M surficial geologic map of the 1:24,000 scale Sloan, MT Quadrangle highlights 3-4 terrace levels in high energy gravels (Blue, Pink, Green) that are inset into varved lakebeds of Glacial Lake Missoula (Salmon). Terrace risers several meters high delineate the gravelly treads.

Lakebeds and till. Varved silts of Glacial Lake Missoula over till of the Mission moraine at Lower Crow Reservoir. Lower Crow Reservoir is located southwest of Ronan, MT some 20 km south of the Polson moraine. Note the large dropstone at contact. Man for scale. 1936 photo by Alden (1953).

Lidar revelations. Moraines and other glacial features are visible in lidar coverage for Mission Valley. Horseshoe-shaped formlines in the landscape were left by Mission ice, which pushed far south of the younger Polson moraine. I'm currently mapping the surficial geology in detail using high-resolution lidar as the basemap.

Unusual currents. J.T. Pardee inspects varved sediments deposited on the floor of Glacial Lake Missoula and exposed near Moiese, MT. 1921 photo in Alden (1953).

Young earthquakes. The Mission Mountains fault and locations of Quaternary scarps and paleoseismic trenches excavated by USBOR (Levish et al., 1993 Figure 64).