2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 8
Presentation Time: 4:00 PM


ANDERSON, David, JOHNSTON, Judith L., NICHOLS Jr, Dennie G. and SUNDELL, Kent A., Geology, Casper College, 125 College Drive, Casper, WY 82601, climbingfrog@bresnan.net

Large boulder deposits form prominent ridges (moraines) and terrace surfaces along streams emanating from the northern and western slopes of Casper Mountain. Shadeline glaciers and climatically related catstrophic flooding are the cause and processes involved in distributing these deposits 1-5 kilometers from the closest Paleozoic and Precambrian bedrock sources. Rapid climatic change, from cold, water-accumulating, glacial stages to warm, melting, interglacial stages is the likely environmental shift that causes juxtaposition and distribution of these perplexing boulder deposits. East/west orientation of Casper Mountain and steep asymmetrically folded and faulted north facing slopes are dominant factors in net ice accumulation and reducing ablation by direct sunlight.

A shadeline glacier is a relatively small (<10 square km), ephemeral (< few thousand years) accumulation of ice in permanent snowfields on generally pole-facing slopes (in the shade of the mountain) that periodically flow downslope in regions often considered to be elevationally too low for classic Alpine glaciation. As shadeline glaciers formed on Casper Mountain, Wyoming's basins were permanently frozen, evidenced by sand wedges and patterned ground, but not accumulating ice cover due to solar ablation and less precipitation.

Field evidence of short U-shaped valleys, moraines, erratics, paleotopography, shape and orientation of diamictons, striated and facetted stones and long distance transport of very large boulders suggest ice movement as shadeline glaciers. Subsequent catastrophic flooding is supported by reworking of moraines, multiple large boulder terraces, steeper boulder terrace gradients, wide flood plains, subrounded clast shapes, terrace sedimentology, and progressive redistribution of some large clasts (but not the largest boulders) downstream from direct glacial deposits. Precise timing of these deposits will be critical in determining if they match the end of the coldest glacial stages (shadeline glaciers) and rapid switching to warm interglacial stages (catastrophic flooding).