Boulder clay

Boulder clay, in geology, is a deposit of clay, often full of boulders, which is formed out of the ground moraine material of glaciers and ice-sheets wherever they are found. It was the typical deposit of the Glacial Period in northern Europe and North America.

Boulder clay is also known as till (German: Blocklehm, Geschiebemergel French: argile à blocaux Swedish: Moränlera; Dutch Keileem).

Boulder clay is distinguished from other clay in that it contains more boulders. The clay is scraped from the top layer of older rock by the movement of a glacier or ice sheet. An ice sheet pushes rocks, boulders and everything else in its path, which in turn wears the rock into silt-like grain, which makes up the clay. Clay formed out of Old Red Sandstone areas 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 until exposed to weather, which transforms it to a brown colour.

Boulder clay is classed with a 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.

...
Wikipedia