Dictionary.net

(Geol.) An important member of the Glacial deposits, Northern drift, Erratics, etc., of the post-Tertiary system. The glacial beds, produced from glaciers, coast-ice, and icebergs, differ in the several parts of England. They comprise the Lower B. clay (a sandy clay, with pebbles and boulders of granite, greenstone, grit, etc.), the Middle drift of sands and gravels, and the Great Upper B. clay. Arctic shells occur in some places. In Scotland, the Till, a dark clay with boulders of old hard rocks, is the chief member.