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Periglacial Soil Wedges East of Glacier National Park

Brown soil wedges follow polygonal crack networks in fine grained sediment (lakebeds) south of Browning, MT. The features formed during the Pleistocene under periglacial conditions in a ground ice region mapped by Murton (2020) and French (2017).

Periglacial wedges are reported in the Lemhi Range of Idaho (Butler, 1984), the Owl Cave-Wasden Site on the Snake River Plain (Dort, 1968; Butler, 1969), Glacial Lake Missoula basin in western Montana (Chambers, 1984; Chambers and Curry, 1989; Levish, 1997; Hanson et al., 2012; Smith, 2014, 2021), glacial deposits east of Waterton-Glacier Parks (Karlstrom, 1990), and terrace gravels near Lewistown, MT (Schafer, 1949). None are as conspicuous as the larger scale, fully-developed networks of fossil ice wedge casts near Laramie, WY (Grasso, 1979; Mears, 1981, 1987), but do resemble features in other high-elevation basins of Wyoming (Nissen and Mears, 1990; Munn and Spackman, 1991; Dillon and Sorenson, 2007).

Soil wedges like these are indicators of a cold, dry, windswept landscape that existed at the end of the last Ice Age. Its probably not correct to call that past landscape "permafrost" (MAT below 0 degC) nor is it proper to call it "tundra" for various reasons. But its pretty close - maybe "cold steppe" is a fitting term. Soil freezing occurred, but it may have been seasonal, not perennial like in the Arctic.

You can tell that these wedges are relict features (no longer actively forming) because they are truncated by a younger loess soil (possibly an Ap?). This area used to be covered by the Two Medicine Lobe of the Laurentide Ice Sheet (called "Keewatin Ice Sheet" in some older USGS publications) and possibly Glacial Lake Cutbank (Calhoun, 1906; Alden, 1932). Not sure which lake the sediments are at the site - may be local deposits. Glacial ice retreated prior to wedge growth.

Photo location is east of Glacier National Park, on the East Front of the Rocky Mountains. Drive south from Browning, MT on Hwy 89 to roadcuts between Two Medicine River and Badger Creek.

Please contact me if you know of other relict soil wedges/fossil ice wedge casts in Montana.

Excerpt of a map in a book that was on the shelf at my relative's Teton Village place. Map modified from one in the book 'The Range' by Sherm Ewing (1990).

Sediment that fills the wedges is not vertically laminated, a pattern of repeated incremental widening, as is common in many Arctic examples.

Location of wedges in terrain formerly occupied by the Two Medicine Glacier. Map by Alden (1932).


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