13.2: 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 then does have substantial erosive power: it 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. The constant abrasion of exposed rock also creates polished bedrock.

Figure \(\PageIndex{1}\): Polished bedrock. Kilarney Provincial Park, Canada (Image Courtesy: Michael Ritter)

Figure \(\PageIndex{2}\): Glacial Striations. Camera Lense cap for scale. (Image Courtesy USGS)

Figure \(\PageIndex{3}\): Glacial Grooves. Grooves in granite bedrock, Kelly Island, Ohio. (Federal Highway Administration)

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 is a jumbled mixture of fine and coarse textured material as shown in Figure \(\PageIndex{4}\). 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 \(\PageIndex{4}\): 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. Kansas and Nebraska in the United States for instance have red quartzite erratics from South Dakota.

Figure \(\PageIndex{5}\): 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 \(\PageIndex{6}\): Sands and fine gravels in stratified glacial outwash near Houghton Lake, Michigan. (Courtesy of Randall Schaetzl, Michgan State University)

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, the Loess Plateau of China, and the European Loess belt are well-known examples of loess topography.

Figure \(\PageIndex{7}\): Loess deposit in Pierre Marquette State Park, Illinois (Source: Michael Ritter)