Major rivers in the region, including the Witham, Welland, Nene and Great Ouse, drain an area of some —five times larger than the fens—into the basin that forms the fens.
Sloj gline Kimmeridge se potaplja zahodno v osnovo Lower Greensand na izpostavljenem območju približno 2 km južno od Ely v jamah Roswell.