Glacial Systems

Geologic work of glaciers

Glacial erosion

Glaciers themselves do relatively little significant erosion because ice is so soft.  Under the weight of an ice sheet thousands of feet thick continental glaciers detach material from the surface by crushing the underlying bedrock. Once the material is loosened from the surface, ice can quarry (also known as plucking) the rock by freezing around and into fractures, then lifting it from the surface. The rock embedded in the ice gouges and smoothes bedrock surfaces by abrasion. Striations are fine scratches left in bedrock by abrasion. At a larger scale, linear grooves are ground into the bedrock in the direction of ice movement.  Chipping of bedrock leaves crescent-shaped marks called chatter marks gouged into the bedrock. The constant abrasion of exposed rock also creates polished bedrock. 

Figure 19.12a Chatter marks in bedrock, Kilarney Provincial Park, Canada

Sun glasses for scale. Image Courtesy: Michael Ritter

Figure 19.12b Polished bedrock

Kilarney Provincial Park, Canada Image Courtesy: Michael Ritter

19.12c Glacial Striations

Camera Lense cap for scale. Image Courtesy USGS

Figure 19.12d Glacial Grooves

Grooves in granite bedrock, Kelly Island, Ohio Source: Federal Highway Administration

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Figure 19.12a-d Features of Glacial Erosion.
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Glacier Transport and Deposition

Glacial drift deposited directly by a glacier is called till. Unlike sediment deposited by running water, till is not sorted by sediment size. Till looks like a jumbled mixture of fine and coarse textured material as shown in Figure 19.10. Fragments of rock encased in ice tend to retain their angular shape, though occasionally the edges are knocked off when rubbing against the bedrock beneath.  Rocks that tumble and roll through a stream are more rounded from striking the bed of a stream channel. 

Figure 19.13 Glacial till in moraine in Whiteshell Park,Canada. Source: USGS

Erratics are boulders whose composition does not match the local bedrock where they are found. Glaciers transported erratics great distances, then deposited them in their current locations as the glaciers retreated. 

Figure 19.14 Giant erratic in Yellowstone National Park

Water melting from a glacier carries sediment with it.  Outwash is stratified drift laid down by water issuing off a glacier. The velocity slows as meltwater runs across the surface causing sediment in the water to deposit. The varying water velocity sorts and layers the drift. Streams running off the edge of the glacier overburdened with sediment form braided streams in outwash deposits.

Figure 19.15 Sands and fine gravels in stratified glacial outwash near Houghton Lake, Michigan. Courtesy of Randall Schaetzl, Michgan State University

Figure 19.16: "Basal Till and Strations"
Courtesy of UBC Virtual Soil Learning Resources