The geology of the Greater Manchester area is dominated by rocks from three main periods of geological time.
There is a general trend of outcrop with the oldest rocks being present in the north-east of the area and the youngest in the south-east. The upland moors of the Dark Peak and southern Pennines to the east of Manchester are formed in the Upper Carboniferous sandstones and shales of the Millstone Grit. This is overlain by the shales, mudstones and thin coals of the Upper Carboniferous Coal Measures which underlie the undulating tract of land to the north of Manchester on which the towns of Oldham, Rochdale, Bolton and Wigan are located.
The outcrop of the Coal Measures also extends southwards down the eastern side Manchester through Ashton-Under-Lyme, Hyde and Hazel Grove. The majority of Manchester and the urban fringe to the south is located on Permian sandstones and red Triassic sandstones and mudstones. Away from the upland areas of the east, much of the area is mantled by thick deposits of till and pockets of sand and gravel deposited by glaciers at the end of the last ice age, some 15,000 years ago.
Carboniferous rocks (354-290 million years old) comprise much of the solid geology of the Greater Manchester area, forming part of the Lancashire-Yorkshire Coalfield, and belong to two main series; the Millstone Grit and the overlying Coal Measures. The hard 'grit' sandstones and intervening shales of the Millstone Grit outcrop to the east of Manchester where they form the upland area of the Southern Pennines and the Dark Peak and the high ground to the north of Rochdale and Bolton.
In these areas the gritstones typically give rise to soil conditions that support heathland and blanket peat. These rocks were deposited as sediments in a coastal environment where large river deltas were building out into the shallow, tropical marine waters that covered much of Britain at this time. Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy land area in which the succeeding Coal Measures were deposited.
The different rates of erosion in the alternating layers of sandstone and shales of the Millstone Grit leads to the extensive rolling moorland being punctuated by vertical cliff-like faces of 'gritstones', such as that at Blackstone Edge, near Rochdale.
In Upper Carboniferous times, the periodic flooding and building of the swamp deltas along the coastline resulted in the deposition of a series of layers of coals (representing the compressed remains of the luxuriant swamp vegetation) interspersed with layers of shale, clay, sandstone and mudstone. These sediments now form the Coal Measures, which overlie the Millstone Grit. Fossils within the various layers of the Coal Measures include plants, marine shells and animals that lived in brackish water conditions.
These fossils provide an indication of the environmental conditions at the time of deposition and show that there were repeated advances and retreats of the shallow sea over the deltas throughout the Upper Carboniferous. The transition to the Coal Measures outcrop is marked by an undulating area of 100-300m height along the eastern fringe of Manchester and Oldham. The presence of coal accounts for the early industrialisation of the area and much of its industrial development. Coal has been worked at depth and by opencast operations at the surface.
The Permian (290-248 million years ago) and Triassic (248-205 million years old) Periods in Greater Manchester are represented by red mudstones and sandstones deposited under arid, desert conditions on a large continental landmass. Sandstones and breccias of Permian age outcrop as a number of north-south trending ridges across north-east Manchester and down to Stockport.
These are overlain by the Lower Triassic sandstones and mudstones of the Sherwood Sandstone Group which consist largely of red, yellow, and brown sandstones that represent the deposits of large braided rivers that crossed the desert plain. These rocks underlie much of central Manchester, and the southern Greater Manchester area and occupy the northern rim of the Permian-Triassic Cheshire Basin.
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.
The main deposit of Quaternary age is till (or boulder clay), which formed in and beneath glaciers and ice-sheets. This covers much of the lower-lying area away from main upland area of the Dark Peak and the South Pennines. Following melting of the ice sheets at the end of the last glacial period, some 12,000 years ago large amounts of gravel and sand were released. These are mainly concentrated in the existing river valleys.
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.