River Irwell - Geology

Geology

The Irwell is all that remains of the shallow seas that covered most of south-east Lancashire in the Late Carboniferous period, when deposits of mud and sand were laid down. During the Permian and Triassic periods, red sandstones were deposited under arid, desert conditions and these became compressed into beds of shales, New Red Sandstone and Manchester marls, alternating with layers of gritstone . The glaciers of the Pleistocene period radically re-shaped the landscape and then retreated, leaving behind deposits of sand, pebbles and boulder clay that formed the fluvioglacial ridges of the lower Irwell Valley. Ashclough, a 14.3-acre (5.8 ha) site which comprises the steep banks of the river between Prestolee and Little Lever, has been designated an SSSI because of its geological interest, primarily because it is the best site in the area displaying the Ashclough marine band and its associated strata. Ashclough is a site of national importance for interpreting the coal measure palaeogeography of Great Britain.