These two pictures show a continental glacier at two time. The top picture shows the glacier just beginning to retreat, and the lower picture shows the glacier after it has retreated some distance.
In the top picture, the glacier has retreated a short distance from its greatest extent. Around its edges, the ice spreads out into long tongues, called glacial lobes. Material carried by the glacier has built a recessional moraine, and a new recessional moraine is forming around the present margins of the glacier. When the glacier retreats quickly, it leaves behind a thin blanket of debris called ground moraine. Melt water from beneath the glacier is flowing outward, carrying a great deal of sediment with it. It forms a network of braided streams that change course frequently as channels clog with sediment and new channels are cut. As the ice retreats, it leaves behind isolated masses called stagnant ice, so called because the ice is not flowing. The brittle outer surface of the ice cracks into crevasses.
In the bottom diagram, the glacier has retreated further. Periods of ice stability when the ice margins stay in one place are marked by recessional moraines, and periods of rapid retreat are marked by a thin cover of ground moraine. The deposits left by the melt water streams in the top diagram now form a flat sandy outwash plain. Retreat of the ice has exposed features formed beneath the ice. Melt water streams flowing in crevasses and tunnels are marked by ridges of sand and gravel called eskers. The sand and gravel are deposits on the former bed of the stream, which was encased in ice. Sometimes cavities in the ice are filled by sand and gravel, and when the ice melts, these deposits are left behind as isolated hills called kames. Drumlins are oval, streamlined hills aligned with the former ice flow. Some seem to form when the ice heaps up and sculpts debris beneath the ice, others may be formed by sudden outbursts of melt water from beneath the glacier. The isolated masses of stagnant ice in the top diagram have melted to leave depressions, which fill with water to become kettle ponds. In the distance, melt water trapped against the ice forms an ice dammed lake.
Created 03 February 2008, Last Update 25 May 2011
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