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 America. Boulder clay is variously known as till or ground moraine (Ger. Blocklehme, Geschiebsmergel or Grundmorane; Fr. argile a blocc1ux, moraine profonde; Swed.