Boulder clay, also known as till, is a type of unsorted glacial sediment composed of clay, sand, gravel, and boulders that were deposited directly by a glacier. It is significant in the study of glacial processes and the reconstruction of past glacial environments. Boulder clay often serves as a key indicator of glaciation in the geological record.
Reference: Benn, D. I., & Evans, D. J. A. (2010). Glaciers and Glaciation. Routledge.