Geologic work of glaciersGlacial erosionGlaciers 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. Episodic movement 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 is the general term applied to materials eroded from the surface and deposited by glaciers. Glaciers transport the embedded material towards the front of the glacier as if they were on a conveyor belt, or is deposit directly beneath the ice. Most material is embedded in the lowest few meters of the glacier and along its sides. Little drift material is lodged in the interior as flow through most of the glacier is laminar, except at the nose where thrust faulting of the ice occurs. When the ice becomes so burdened by its load of soil and rock fragments, it deposits the mixture of fine and coarse textured material in place as glacial till. Till is distinguished by its lack of sorting.
At the margin of the glacier, massive amounts of rock debris are deposited as the ice stagnates and melts in place. The meltwater flushes through the accumulated debris, spreading drift ahead of the decaying glacier as stratified drift. |
Figure
FS.6 Glacial till on Mt.
