Clastic Dikes: Dispelling a Periglacial Origin

What Does Periglacial Mean?

"Periglacial" is a term that packs a punch. Like "tundra" or "permafrost", it stands in for a full suite of cold-climate features, biophysical processes, and conditions. Periglacial regions are those that lie just beyond areas covered by glacial ice. They are swaths of cold-affected ground that often fringe glaciated terrain where Mean Annual Temperature (MAT) ranges between -2 to -5 degC. Frozen ground is the common to all periglacial environments, past or present (Pewe, 1973; French, 2017), but glaciers are not. Today, periglaciation is limited to the high latitudes (i.e., Alaska, Greenland, Svalbard, northern Eurasia) and high elevation mountains (Himalaya, Andes, etc.).

Active ice wedges and fossil ice wedge casts are perhaps the only a sure indicator of current or relict periglaciation (Black, 1976), but other cold-related features commonly found in these cold regions. Stone circles, cryoplanation terraces, thermokarst basins, frost-shattered bedrock, cryoturbated soils, and gelifluction deposits are few examples (Murton, 2021).

Murton (2021) uses four criteria to identify modern and relict periglacial landscapes in North America (3.5 Ma to present). His criteria include (1) persistent cold, (2) extraglacial location, (3) ice-rich substrates, (4) significant aggradation of sediment and ground ice. Murton's empirical and geologically-based system improves upon earlier efforts to map and classify periglacial reigons (Tricart and Cailleux, 1972; Mikhailov, 1981; Budel, 1982; Karte, 1982). By Murton's criteria, the Columbia Basin during the Last Glacial Maximum (LGM) would have met just 1 of his 4 criteria (extraglacial location). Today, it would still meet just that one.

Incipient frozen ground. Frost cracks (soil cracks) likely caused by cold penetrate glacial till at Foster Creek, WA. The features are not abundant in roadcuts and stream banks of the area, but are found occasionally. My photo taken October 2017.

Eastern Washington's Missing Periglacial Zone

Soils in Eastern Washington have never developed ground ice. Fossil ice wedge casts, regularly found in areas formerly occupied by Pleistocene ice (i.e., New Hampshire) or impacted by deep cold during the last Ice Age (i.e., Laramie, WY), are absent in Eastern Washington. Palouse loess and scabland deposits contain abundant Pleistocene-age phytoliths, rodent burrows, and insect burrows - features that require the absence of ground ice. Rodents repeatedly recolonized the landscape between megafloods. Soil surveys completed for the entire state contain detailed information on unconsolidated Pleistocene and Holocene surficial sediment (soil parent materials). The soil survey of the Colville Indian Reservation (NRCS, 2002) is a good example of a detailed soil survey, one of the nation's most detailed reports of its kind (178 soil map units, >28,000 soil polygons with an average size of 48.6 acres). Dozens of 1:24,000 scale map sheets cover 1.36 million acres of Okanogan and Ferry Counties, including both glaciated and unglaciated terrain. Project staff were capably managed by Steve Campbell. Mapping was conducted at a scale fine enough to reveal relict ground ice features, however, no mention of relict ground ice features, frost stirring, or gelifluction is made. No such features are reported in similar reports for nearby Okanogan County (NRCS, 1980/2010), Chelan County (USDA, 1975), Douglas County (NRCS, 1981/2008), Grant County (USDA, 1984), or Lincoln County (USDA, 1981).

No Tundra in Eastern Washington

Soils in Eastern Washington have never supported a tundra plant community. Tundra biomes are identified by their lack of trees. Tundra plants, common in today's Arctic, are adapted to cold conditions, short growing seasons, shallow soils, and rooting zones limited by shallow bedrock or permafrost. Pleistocene mammoth in the Horse Heaven Hills were nourished by steppe-grassland forage (Barton, 1999), not tundra plants. Pollen data from Columbia Basin lake cores reveal the presence of certain cold-tolerant, low-growing species, but the region never lost its tree component entirely (Blinnikov et al., 2002; Whitlock et al., 2006).

Washington wedges. Small collapse feature filled with rubble at Banks Lake in Upper Grand Coulee, WA. My photo c. 2005.

Narrow Periglacial Zone at Former Ice Sheet Margin

The southern terminus of late Wisconsin glacial ice is well mapped across northern Washington (Cheney, 2016, Fig 17.1), but a corresponding periglacial zone is all but absent. Murton (2021, Fig. 3) depicts a narrow permafrost zone south of the Okanogan Lobe's terminal moraine, indicating the ice margin and the southern limit of permafrost occupied essentially the same position. Unfortunately, he provides no evidence of periglaciation along the Withrow Moraine (see French and Millar, 2013; French, 2017). By contrast, abundant relict periglacial features populate a 200 km-wide zone south of the Laurentide Ice Sheet (Pewe, 1983; Clark and Ciolkosz, 1988).

The absence of a more typical periglacial zone between Pateros and Spokane is difficult to explain. Scant evidence that I have been able to document consists of crude frost cracks along Banks Lake (varved silt-clay, 478 m elevation), at Leahy Junction (varved silt-clay, 612 m), at Hunters (lacustrine silts, 393 m), and at Foster Creek (lodgment till, 384 m). Perhaps the ice sheet did not remain at its farthest-advanced position long enough for significant ground ice to accumulate. Glacial action may have buried or over-ridden periglacial features formed when the margin was farther north.

Narrow periglacial. A narrow periglacial zone (pink) is depicted south of the Cordilleran Ice Sheet margin (white). Modified from Murton (2021).

Other Evidence

Cirque elevations - contours project far above the crests of Yakima Fold Belt ridges (Pierce, 2003, Fig. 1).

Clastic dikes - Clastic dikes in Missoula flood rhythmites are not fossil ice wedge casts (Jenkins, 1925; Cooley, 2020).

Frost shattering - Frost shattering in Columbia River Basalt, exposed over thousands of square kilometers, was not unusually intense (i.e, Pidwirny, 2006).

Loess and mima mounds - Thick loess and mima mound fields, often erroneously attributed to periglacial processes. Neither is not diagnostic of periglacial landscapes. Silt mounds are found from Alaska to Texas and certain mounds in south-central Washington are Holocene age. Thick loess deposits in North America are found in formerly glaciated, unglaciated, and desert regions. Busacca et al. (2004) summarize,

Ultimately, each eolian system has been driven by different controls and there are few synchronous units or soils that span the continent. Each region responded differently to glacial/interglacial conditions as a result of variable climate and vegetative cover, sediment character, and supply.

Ice Age wind patterns - Katabatic winds may have been mitigated by maritime weather systems off the Pacific or warm air masses moving northward out of the central Columbia Basin. If an anticyclonic wind pattern did develop over the former ice sheet at LGM, as Sweeney et al. (2004) postulate, it was too weak or infrequent to defeat the prevailing onshore flow (Muhs and Bettis, 2000). Northeastward transport of loess driven by southwesterly winds has remained consistent over most of the past 2 million years (McDonald et al., 2012).

Ice wedges in an aggrading surface. Syngenetic ice wedges form in polar regions in places where new sediment is periodically added to the ground surface. Vertically-sheeted clastic dikes in Ice Age floodways of the Columbia Basin are not syngenetic ice wedge casts. See this post for more information:

Relict Ground Ice Features in the Rocky Mountains

Permafrost wedges and soil involutions are reported in the Lemhi Range of Idaho (D.R. Butler written communication and photos), the Owl Cave-Wasden Site on the Snake River Plain (Dort, 1968a; Miller and Dort, 1978; Butler, 1984), Glacial Lake Missoula basin in western Montana (Chambers, 1971; Chambers and Curry, 1989; Levish, 1997; Hanson et al., 2012; Smith, 2014, 2021), and terrace gravels near Lewistown, MT (Schafer, 1949).

Subaerial cracks? Frost cracks, some filled by collapse breccia, repeat dozens of times in varved beds at the Rail Line Section near Missoula, MT (Hanson et al., 2012; Smith and Hanson, 2014; Smith, 2021). Cracks are associated with weathered surfaces that appear to represent subaerially-exposed surfaces (drained surfaces) in the Glacial Lake Missoula basin. My photo taken August 2021.

Leave Town, Discover Montana

Recently, I discovered fossil soil wedges following a polygonal crack pattern on the Two Medicine Creek-Badger Creek divide east of Glacier National Park. The wedge networks are smaller and less conspicuous than the fossil ice wedge networks in high-elevation basins of Wyoming (Grasso, 1979; Mears, 1981, 1987; Nissen and Mears, 1990; Munn and Spackman, 1991; Dillon and Sorenson, 2007). The location is >525 km east of Withrow, WA. The climate at Browning is continental with modern winter temperatures regularly falling 10-30 degrees below zero for long periods. It was even colder during the Pleistocene. See this post for more information:

Montana wedges. Soil wedges follow polygonal crack networks in Pleistocene lake beds east of Glacial National Park. Roadcut is located along Hwy 89 between Two Medicine River and Badger Creek, south of Browning, MT. Winter conditions were then and are today much colder on Montana's East Front than in Washington's Columbia Basin, yet only small soil wedges formed during the last Ice Age east of Glacier National Park. My photo taken January 2021.

Implications for Clastic Dikes in Eastern Washington

The best argument for a periglacial origin for clastic dikes in Eastern Washington builds on the fact they are vertically sheeted and wedge shaped (Black, 1979). The Arctic geomorphologist, Robert F. Black (USGS and University of Connecticut), examined dikes in the field, weighed the pros and cons of a ground ice origin, and ultimately concluded the dikes are not periglacial features,

Previously proposed theories...earthquakes, desiccation, deep frost cracking, thermal contraction cracking of permafrost, and upward injection of groundwater are not considered primary modes of formation...The bulk of material filling most observed fractures came from above during aperiodic and repeated widening and concurrent filling (under an aqueous environment)...A loading hypothesis from catastrophic scabland floods is outlined as a possible cause for many typical clastic dikes...

Black ultimately settled on a "multigenetic" interpretation for the dikes, an idea that traces directly to an article by Dionne and Shilts (1974), specifically their Table 1, which lists 31 articles on clastic dikes of various origins. At that time, Dionne was in Quebec, Shilts in Ontario, and Black in Connecticut; all East Coast geomorphologists employed by government agencies and knew each other. Black's "mulitgenetic" finding was adopted by a cadre of then young, now retired Hanford geologists who assisted him with his field investigation (J.A. Caggiano, G.V. Last, J.T. Lillie, A.M. Tallman). A careful reading of Black, however, reveals a lengthy discussion and a convincing argument for one origin: floodwater loading.

Black's thoughts are well summarized in Woodward-Clyde Associates (1981),

Black (1979) hypothesized that hydraulically dammed late Pleistocene floodwater, which repeatedly covered the area, was responsible for the formation of the fractures - for the aperiodic widening of these cracks - and was the primary source of material that filled the cracks. Sudden loading by floodwater on a ground surface whose ground-water level was not close to the surface produced stresses that were irregularly distributed. These stresses induced cracking of the ground, which would have allowed turbid water to enter. Sediment in the water would at first have been filter pressed against the walls of the crack, creating the "clay skins". Fractures could have been widened as the load increased or as shear resistance decreased with increasing pore pressure. Continued widening of the crack would have permitted coarser sediment to enter. The flow of sediment-laden water along the length of a crack would have produced the foreset-bedding structures frequently seen.


Conditions in the Late Pleistocene Columbia Basin were not periglacial as some have suggested (i.e., O'Geen and Busacca, 2001). At its coldest, the landscape was "tundra-like" (Cooley, 2008), but lacked ground ice. Perhaps the region at that time is best described as a "cold steppe" (Spencer and Knapp, 2010, p. 50) with sagebrush and forest refugia that supported species,

...found in alpine and sub-alpine valleys in the [present-day] Cascade Mountains of, moist, open-park conditions...consistent with the presence of continental ice to the north.

Jocko Valley. A small periglacial wedge in deposits of Glacial Lake Missoula in the Jocko Valley near Ravalli, MT. My photo taken Dec 2017.


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