Boulder clay
![WWII pillbox on eroding Boulder Clay, Filey Bay, North Yorkshire.](/Images/godic/202501/01/Boulder_Clay_Filey_Bay_pillbox4856.jpg")
![Boulder clay cliffs in Gwynedd with Dinas Dinlle in the background](/Images/godic/202501/01/Boulder_Clay_Cliffs_-_geograph.org.uk_-_2501104856.jpg")
Boulder clay, in geology, is a deposit of clay, often full of boulders, which is formed out of the ground moraine material of glaciers and ice-sheets wherever they are found. It was the typical deposit of the Glacial Period in northern Europe and North America.