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Boulder clay, in geology, is a deposit of clay, often full of boulders, which is formed in and beneath glaciers and ice-sheets wherever they are found, but is in a special sense the typical deposit of the Glacial Period in northern Europe and North America. Boulder clay is variously known as till or ground moraine (Ger. Blocklehme, Geschiebemergel or Grundmoraene; Fr. argile a blocc1ux, moraine profonde; Swed. Krosstenslera). It is one of the group of poorly sorted materials described by the non-genetic term diamicton. It is usually a stiff, tough clay devoid of stratification; though some varieties are distinctly laminated. Occasionally, within the boulder clay, there are irregular lenticular masses of more or less stratified sand, gravel or loam. As the boulder clay is the result of the abrasion (direct or indirect) of the older rocks over which the ice has travelled, it takes its color from them; thus, in Britainmarker, over Triassic and Old Red Sandstone areas the clay is red, over Carboniferous rocks it is often black, over Silurian rock it may be buff or grey, and where the ice has passed over chalk the clay may be quite white and chalky (chalky boulder clay). Much boulder clay is of a bluish-grey color where unexposed, but it becomes brown upon being weathered.

The boulders are held within the clay in an irregular manner, and they vary in size from mere pebbles up to masses many tons in weight. Usually they are somewhat oblong, and often they possess a flat side or sole; they may be angular, sub-angular, or well rounded, and, if they are hard rocks, they frequently bear grooves and scratches caused by contact with other rocks while held firmly in the moving ice. Like the clay in which they are borne, the boulders belong to districts over which the ice has travelled; in some regions they are mainly limestones or sandstones; in others they are granite, basalts, gneisses, etc.; indeed, they may consist of any hard rock. By the nature of the contained boulders it is often possible to trace the path along which a vanished ice-sheet moved; thus in the glacial drift of the east coast of Englandmarker many Scandinavian rocks can be recognized.

With the exception of foraminifera, which have been found in the boulder clay of widely separated regions, fossils are practically unknown; but in some maritime districts marine shells have been incorporated with the clay.

A classic example of boulder clay can be seen at the rapidly eroding cliffs of Hornseamarker, situated along the Holderness coast in East Yorkshire.

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