Shallow seas covered most of south-east Lancashire in the Upper Carboniferous period when deposits of mud and sand were laid down. These later became beds of shales and sandstones alternating with layers of gritstone. There are also beds of New Red Sandstone and Manchester Marls. The glaciers of the Pleistocene epoch further shaped the landscape and then retreated, leaving behind deposits of sand, pebbles and boulder clay that formed the fluvioglacial ridges of the Irwell Valley. Ashclough, a 5.8 hectare (14.3 acre) site which comprises the steep banks of the river between Prestolee and Little Lever, has been designated a Site of Special Scientific Interest (SSSI) because of its geological interest, primarily because it is the best site in the area displaying Ashcough Marine Band and its associated strata. Ashclough is a site of national importance for interpreting the coal measure paleogeography of Great Britain.