19.2: 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 .
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 \(\PageIndex{5}\). 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.
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.
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.
Video : "Basal Till and Strations" (Courtesy of UBC Virtual Soil Learning Resources)
Loess is principally wind-deposited silt, much of which was eroded from Pleistocene glacial sediments. Vast deposits of loess are spread across the upper Midwest United States, with significant deposits along major rivers like the Mississippi. The sediment is deposited by glacial meltwater then eroded by strong winds. The Loess Hills of Iowa and Loess Plateau of China are two well-known examples of loess topography.