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Filled glacial lakes are another form of outwash. In glacial terrain, lakes will form as a result of glacial or fluvio-glacial deposition, in basins produced by glacial erosion, or in ice margin lakes (lakes forming against the ice). These lakes are often filled quickly as excessive bed loads of glacial melt water provides large volumes of clay, silt, sand, and gravel. At a distance from the glacier, clay and silt make up a large portion of the stream load so that some of these lake deposits may consist of alternating dark and light layers called varved clays. Each pair (light and dark) consists of a layer of predominantly fine silt or sand overlain by a darker clay rich layer. The silt and sand may be due to summer deposition while the clay is due to winter deposition. Glacial lake deposits can reach considerable thicknesses (many hundreds of feet).
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