A Brief Geological History of Matlock & the Peak District
First Ideas
Matlock sits at the south-eastern edge of the Peak District. The simple story of the shape of the Peak District with its ring of gritstone/sandstone surrounding a core of limestone is that it was uplifted as a dome (anticline) and the central core of gritstone / shale was eroded away, revealing the limestone (below, left). The outer (Dark Peak) area is composed of gritstones and shales. Further out, are the coal measures of the Nottinghamshire and Staffordshire Coalfields. At the boundary of limestone and gritstone, Matlock has very different scenery to the south (limestone) and to the north (gritstone). There has been extensive quarrying of both limestone and gritstone in the area and minerals, in particular, lead (galena) have been mined since Roman times. It was assumed that the older basement of the dome was a granite pluton like those in Cornwall, and that the heat of the cooling mass of molten granite had driven minerals up from depth and deposited them in fissures in the limestone above. However the true story is more complicated….
More Recent Evidence
This simple picture was blown apart by boreholes drilled in the 1970s at Woo Dale (near Buxton) , Eyam (5 miles north of Bakewell) and Caldon Low (near Ashbourne)(1). Whilst the simple picture shown above denotes broadly similar depths of limestone across the dome, the boreholes showed a seven-fold variation in the thickness of the limestone. At Caldon Low in the south and Woo Dale in the west, the thickness was only about 250m, but at Eyam it was over 1800m. Furthermore the basement rock was different in each case and none of them was granite.
Subsequent geophysical work by BGS demonstrated the presence of massive faults, so the limestone laid down in Carboniferous times had not been laid on an even basement. Studies suggest that the rocks have since been subjected to stretching along a north-east / south-west axis, followed by twisting with the northern part being rotated against the southern part. Then it suffered compression along the same north-east / south-west axis. Finally in the later Carboniferous, there was an extension along the north-west / south-east axis. With such a violent history, it is not surprising that the limestones in this area contain many cracks and faults into which minerals could later be deposited. This final extension does explain why most of the major mineral veins lie in a broadly WNW / ESE direction, although there are numerous smaller one at right angles. However this still does not explain where the minerals came from and how they got there.
Even in the earlier Carboniferous, when the limestone was being laid down, and when Matlock was situated just south of the Equator and enjoying a Caribbean-like climate, life was not entirely peaceful as the next section shows.
High Tor and Matlock Gorge
High Tor is the dominant feature towering above the Derwent Valley.
It is a classic example of a limestone reef knoll that was laid down in the shallow tropical seas of the Carboniferous Era. It is difficult to imagine as the summit of High Tor now stands an impressive 205m / 673 ft above sea level, however any visitor expecting a Caribbean climate has come about 330 million years too late. The cliff itself is about 300 ft high, one of the largest in England.
The reef was largely algal mud in origin rather than coralline, so contains few fossils. A detailed discussion about its formation can be found in an article(2) by Peter Gutteridge for East Midlands Geological Society. [Read what early geologists made of High Tor]
It had a deeper water basin to the north, and to the south, where the Derwent now flows, and there were numerous volcanic islands, as is shown in Trevor Ford’s diagrammatic map(3) of the area. Although these volcanoes were probably not of the scale of the present day Vesuvius, the lava flows of fluid basalt extended across the floor of the lagoon. These Matlock lavas reach the surface at Masson Hill. Lava can also be seen in the entrance passage to Rutland Cavern(3) in Matlock Bath. At Masson Hill, the lava is about 100 m thick(4)
For information about how the Matlock Gorge was formed, see the next section on the River Derwent
The River Derwent
The source of the Derwent is on Bleaklow in the Dark Peak. In cutting eastwards and then southwards through relatively softer shales, the Derwent “captured” flows from other rivers. At Rowsley, it is joined by the River Wye. The valley widens to a broad floodplain and the river meanders until it reaches Matlock, where there is a marked change in its profile. The limestones of Pic Tor and High Tor are far more resistant and the river is confined within a narrow gorge between Matlock and Cromford. The reasons why it should flow here rather than taking the easier course through the softer shales of the Starkholmes Valley to the east is still a matter of debate(5).
The conventional explanation is that it is an example of superimposed drainage; that means that the river course was determined much earlier while the area was still covered by Mesozoic deposits (all of these and later deposits have been completely removed, so that the surface is almost entirely of Carboniferous age)
