Roswell Pits

Roswell Pits is an 8 hectare nature reserve on the eastern outskirts of Ely in Cambridgeshire. It is managed by the Environment Agency. It is part of the Ely Pits and Meadows Site of Special Scientific Interest ) and Geological Conservation Review site. The SSSI designation for both biological and geological interest. The site was formerly managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire.
The pits were a source of gault, an impervious clay used to maintain river banks in the low-lying regions of the South Level of the Fens. Following the re-routing of the rivers in the region by Cornelius Vermuyden and his Adventurers in the 1650s, to more effectively drain the Fens, the peaty soils began to dry out and shrink. As the land surface sunk below the levels of the rivers, it became important to maintain the banks with something impervious to water, to prevent seepage into the newly drained agricultural land, and to prevent collapse of the banks and flooding of the land in times of heavy rainfall. Roswell Pits were an ideal source of this material, as they were located adjacent to the River Great Ouse, and boats could take the bulky material directly to the banks being maintained.

Wikipedia