Geological opinion is still divided on the subject of the formation of the Boulder-clay. By some it is held to be the sole work of the ice, and accumulated beneath it; by others it is looked upon as a marine deposit, though deriving its materials from the grinding action of the ice-sheet or glacier. Some geologists, again, hold that the “Till”—as distinct from the Boulder-clay— was formed beneath the ice, but that the latter is a marine deposit laid down in glacial seas.