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.

Names

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

Description

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.