Most of Manchester, and its suburban fringe to the south, is located on Permian sandstones and red Triassic sandstones and mudstones, overlain by thick deposits of till and pockets of sand and gravel deposited by glaciers at the end of the last glacial period, some 15,000 years ago. The oldest rocks, from the Upper Carboniferous period, are sandstones and shales of Millstone Grit present as outcrops and uplands in the north-east of Greater Manchester, such as the upland moors of Dark Peak and South Pennines to the east and northeast of Rochdale, Oldham and Stalybridge. These rocks are overlain by shales, mudstones and thin coals of Coal Measures upon which the towns of Oldham, Rochdale, Bolton and Wigan are located. An outcrop of Coal Measures extends southwards down through Tameside and into Hazel Grove.
There are currently 20 sites in Greater Manchester that have been designated as Regionally Important Geological/ Geomorphological sites (RIGS). They are also known as Local Geological Sites (LGS) or simply Geological Sites. They were chosen on the basis of their educational value for students of all ages, their historical value in the study of earth sciences, and their aesthetic value in the landscape, especially in promoting public awareness of geology.