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BOULDER CLAY

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Originally appearing in Volume V04, Page 320 of the 1911 Encyclopedia Britannica.
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BOULDER See also:CLAY , in See also:geology, a See also:deposit of clay, often full of boulders, which is formed in and beneath glaciers and See also:ice-sheets wherever they are found, but is in a See also:special sense the typical deposit of the Glacial See also:Period in See also:northern See also:Europe and See also:America. Boulder clay is variously known as " till " or " ground See also:moraine " (Ger. Blocklehme, Geschiebsmergel or Grundmorane; Fr. argile a blocaux, moraine profonde; Swed. Krosstenslera). 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 See also:sand, See also:gravel or See also:loam. As the boulder clay is the result of the See also:abrasion (See also:direct or indirect) of the older rocks over which the ice has travelled, it takes its See also:colour from them; thus, in See also:Britain, over Triassic and Old Red See also:Sandstone areas the clay is red, over Carboniferous rocks it is often See also:black, over See also:Silurian See also:rock it may be See also:buff or See also:grey, and where the ice has passed over See also:chalk the clay may be quite See also:white and chalky (chalky boulder clay). Much boulder clay is of a bluish-grey colour where unexposed, but it becomes See also:brown upon being weathered. The boulders are held within the clay in an irregular manner, and they vary in See also:size from See also:mere pellets up to masses many tons in See also:weight. Usually they are somewhat oblong, and often they possess a See also:flat See also:side or " See also:sole "; they may be angular, sub-angular, or well rounded, and, if they are hard rocks, they frequently See also:bear grooves and scratches caused by contact with other rocks while held firmly in the moving ice. Like the clay in which they are See also: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 See also:granite, basalts, gneisses, &c.; 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-See also:sheet moved; thus in the Glacial See also:drift of the See also:east See also:coast of See also:England many Scandinavian rocks can be recognized.

- With the exception of See also:

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. See GLACIAL PERIOD; and See also:GLACIER.

End of Article: BOULDER CLAY

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