(geology) A geological deposit of clay, often full of large stones and boulders, which is formed out of the ground moraine material of glaciers and ice-sheets.
1959 June, J. F. Oxley, D. R. Smith, “The Nottingham-Kettering line of the L.M.R.”, in Trains Illustrated, page 319:
The geological formation here is boulder clay and lias, and the original intention to keep the railway in open cutting had to be abandoned, as this would have resulted in 90 ft. deep walls through unstable terrain.
