19.8: How Are Glacial Sediments Unique?

How Are Glacial Sediments Unique?

As glaciers flow, they plow up or push aside rock and debris, which is then left behind when the glacier recedes. As large glaciers retreat, the underlying ground surface is typically abraded of most materials, leaving scars and debris on the underlying bedrock surface. Glacial ice is responsible for a large amount of erosion, and a lot of sediment can be carried on top of (supraglacial), within (englacial), and beneath (subglacial) ice. When sediment is left behind by a melting glacier, it is called glacial till Links to an external site. (Figure 19.13).

Till is generally poorly sorted, angular to subrounded, with grain sizes ranging from clay and silt to pebbles and boulders. The lithified rock is referred to as tillite Links to an external site. or, more generally, a diamictite Links to an external site.; both signify a rock that includes a wide range of clast sizes (Figure 19.14).

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When glaciers retreat, they may leave behind large boulders of rock types that do not match the local bedrock, called glacial erratics Links to an external site. (Figure 19.15). Really small clay-sized sediment frequently forms glacial or rock flour Links to an external site., which can cause glacial lakes and water bodies to have a milky appearance.

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Often, the sediment carried within the ice affects the underlying bedrock, or even other sediment carried by the glacier. Glacial striations Links to an external site., scratches, or grooves can be carved into the bedrock, or on the cobbles and boulders carried by the ice (Figure 19.16).

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Additionally, glacial polish Links to an external site., produced by fine-sediment carried by the glacier, creates an almost mirror-like finish to the bedrock (Figure 19.17).