Glacial Landforms

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Glacial landforms are deposits of sediments produced by the advance and retreat of glaciers. As a glacier forms and advances, large amounts of rock and soil are picked up and incorporated into the base of the ice. In alpine glaciers, erosion along valley walls may also contribute sediment build-up on the top of the glacier. The continual flow of the glacier carries these materials forward until they reach the end of the glacier, where they are deposited when the toe of the glacier melts away. If the sediments are deposited directly from the ice, they are called till. Till consists of a range of unconsolidated and usually unstratified materials in a range of sizes, from clay to large boulders. Sediments may also be transported and deposited by glacial melt water and are termed glaciofluvial deposits.

A distinct deposit of till is called a moraine. Because till is deposited more or less as it is turned out by a glacier, the resulting moraines tend to be irregularly and randomly shaped mounds and hills. A variety of moraine types is distinguished. A moraine that is deposited at the toe of a glacier is an end moraine. Till deposited along the edge of a glacier is a lateral moraine. The furthest extent of a glacier before it retreats is marked by a specific type of end moraine called a terminal moraine. Intermittent halts in glacial retreat may produce recessional moraines.

If a portion of a glacier melts in place, leaving all the accumulated sediment in place, it produces a till plain. Large isolated boulders that have been transported and deposited by glaciers are called erratics.

A glacier can also produce more regularly shaped land-forms as well. Eskers are snakelike deposits of sand and gravel deposited in ice tunnels at the base of a glacier formed by flowing streams. Because the sediments in an esker were deposited by running water, they tend to be better sorted than till and can be layered. A kettle is a circular depression, often filled by a lake, that is produced when a large block of ice is detached from a glacier and is subsequently surrounded by till. The ice eventually melts, leaving the depression or lake in its place. Drumlins are elongated, asymmetric mounds of till that resemble a teardrop. They are deposited by a retreating continental glacier with the tapered end indicating the direction of retreat. Kames are conical mounds of sediment deposited where a stream exits a glacier. Outwash plains are large areas of well-sorted glaciofluvial sand deposited beyond the end of a glacier by numerous converging streams of meltwater.

Glaciation; Ice Ages