Boulder-clay, a clay containing boulders or fragments of various other rocks. Boulder-clays are of Pleistocene age, being either marine and stratified, in which case they, or the boulders they contain, are the result of floating ice, or terrestrial and unstratified, when they represent the ground-moraine of a glacier or ice-sheet. The boulders range in size from mere grit up to masses weighing many tons, the latter in Britain being more frequent in the north. From the abundance of pellets of a hard chalk, mainly derived from Lincolnshire, much of the boulder-clay of Suffolk, Essex, Hertfordshire, etc., is almost white and is known as the chalky boulder-clay. In Scotland boulder-clay is commonly known as till. Gravels and sands are commonly associated with the clay, sometimes as mere local patches lenticular in form, and sometimes more extensive. Derived fossils, often ice-scratched, occur in the clay, and flint-implements have apparently been found under some layers of it; but the attempts to subdivide it and to correlate its divisions chronologically have not as yet been successful. The clay is sometimes remarkably contorted, as at Cromer and Sudbury, and may enclose large detached masses of older formations, and this disturbed character frequently extends to the upper part of the underlying rocks, suggesting a ploughing action of ice driven over the surface with enormous force. Boulder-clay varies in thickness from 80 or 90 feet downwards, being generally thinner on mountain-slopes. It occurs extensively in Scotland, England, north of the Thames, Scandinavia, North Germany, Northern and Central Russia, and in the northern half of North America.