Glacial Deposits

Load. An advancing ice sheet carries an abundance of rock that was plucked from the underlying bedrock; only a small amount is carried on the surface from mass wasting. The rock/sediment load of alpine glaciers, on the other hand, comes mostly from rocks that have fallen onto the glacier from the valley walls. The various unsorted rock debris and sediment that is carried or later deposited by a glacier is called till. Till particles typically range from clay‐sized to boulder‐sized but can sometimes weigh up to thousands of tons. Boulders that have been carried a considerable distance and then deposited by a glacier are called erratics. Erratics can be a key to determining the direction of movement if the original source of the boulder can be located.

Features left by valley glaciers and ice sheets. Moraines left by valley glaciers are shown in Figure 1, and features left by a receding ice sheet are shown in Figure 2. Moraines are deposits of till that are left behind when a glacier recedes or that are carried on top of alpine glaciers. Lateral moraines consist of rock debris and sediment that have worked loose from the walls beside a valley glacier and have built up in ridges along the sides of the glacier. Medial moraines are long ridges of till that result when lateral moraines join as two tributary glaciers merge to form a single glacier. As more tributary glaciers join the main body of ice, a series of roughly parallel medial moraines develop on the surface of main glacier.

Figure 1     

Moraines

Figure 2

Landforms Produced After Ice Recession



An extensive pile of till called an end moraine can build up at the front of the glacier and is typically crescent shaped. Two kinds of end moraines are recognized: terminal and recessional moraines. A terminal moraine is the ridge of till that marks the farthest advance of the glacier before it started to recede. A recessional moraine is one that develops at the front of the receding glacier; a series of recessional moraines mark the path of a retreating glacier.

A thin, widespread layer of till deposited across the surface as an ice sheet melts is called a ground moraine. Ground moraine material can sometimes be reshaped by subsequent glaciers into streamlined hills called drumlins, long, narrow, rounded ridges of till whose long axes parallel the direction the glacier traveled.

As a glacier melts, till is released from the ice into the flowing water. The sediments deposited by glacial meltwater are called outwash. Since they have been transported by running water, the outwash deposits are braided, sorted, and layered. The broad front of outwash associated with an ice sheet is called an outwash plain; if it is from an alpine glacier it is called a valley train. Kames are steep‐sided mounds of stratified till that were deposited by meltwater in depressions or openings in the ice or as short‐lived deltas or fans at the mouths of meltwater streams. The rapid build‐up of sediments can bury isolated blocks of ice. When the ice melts, the resultant depression is called a kettle. Kettle lakes, common in the upper Midwest of the United States, are bodies of water that occupy kettles.

Eskers are long, winding ridges of outwash that were deposited in streams flowing through ice caves and tunnels at the base of the glacier. Generally well sorted and cross‐bedded, esker sands and gravels eventually choke off the waterway.

The great volume of meltwater often results in the formation of glacial lakes between the end moraines and the retreating glacier front. The sediments that form at the bottom of the lake consist of fine‐grained silt and clay that have an alternating light‐dark layering. A varve consists of one light‐colored bed and one dark‐colored bed that represent a single year's deposition. The light‐colored layer is mostly silt that was deposited rapidly during the summer months; the dark layer consists of clay and organic material that formed during the winter. The age of a glacial lake can be determined from the number of varves that have formed on the lake bottom.