THE DERBYSHIRE PEAK DISTRICT

The geological story in the Peak District begins some 325 million years ago in the early Carboniferous when the British Isles were just a few degrees south of the Equator. A shallow tropical lagoon had formed floored by fine shifting sediments of mud formed by the shells and bodies of countless sea creatures. Forests of sea-lilies waved their feathery arms in unison with the currents while coiled ammonites used jet propulsion to catch their prey. Near the edges of this lagoon, which stretched 25 miles long by 10 miles wide, forests of sea-weeds were trapping and building up the skeletons of thousands of sea creatures eventually to form fringing-reef mounds. Beyond the reefs, a similar process of accumulation was going on, only more slowly, in the deeper ocean.

These vast sediments (608m thick in places) were eventually to form the limestones of the White Peak, the bedrock on which subsequent sediments were laid during the Carboniferous. Evidence of the sea-lilies can be found as the fossilised crinoids or 'Derbyshire screws' found throughout the White Peak, but particularly in Dovedale where erosion has made them stand proud of the rock.

In Victorian England, the 'crinoidal' limestone was highly sought after for ornamental purposes, providing stone for fireplaces, floors, washstand tops and window sills and surrounds. When it was cut and highly polished, it showed interesting and varied cross-sections of the stems.

The accumulations of reef limestones are punctuated in the Peak District by periods of volcanic activity during which the entire lagoon floor was covered by bubbling viscous lava from an undersea vent. This lava formed a dark green-blue rock called basalt, often containing calcite-filled gas bubbles. These ancient lavas reach 10-15m thick in the Peak District and are known locally as 'toadstones'. Notable outcrops are found in Millers Dale and Tideswell Dale close to the Ravenstor Hostel.

Not only did vulcanicity leave behind thick layers of basalt, but the associated hydro-thermal veins left vast deposits of galena (lead ore), fluorspar (blue john), barytes, calcite and copper, all of which have been extensively mined. Of these minerals, lead has proven to be the most important in terms of mineral wealth it having been mined since Roman times and evidence for this is found everywhere in the White Peak. Indeed it was the search for lead which led miners to discover the Castleton 'blue-john' quite by chance.

Whilst thick accumulations of limestone were being formed in the Peak District, the rocks of the old Laurentian continent were being folded and uplifted in the area which was to become the Scottish Highlands. The uplifting of these mountains gave rise to large rivers which flowed southwards into the shallow sea which was covering the Peak District and much of the present Pennines. These rivers brought sediments of fine sand, grit, silty mud and pebbles to form large fan-shaped deltas over-riding the limestone beds of the lagoon and reefs. The area would have resembled the present-day Gulf of Mexico, with the muddy Mississippi pushing forward its vast delta. Occasionally earth movements would tilt or raise the highlands to cause different distribution patterns in the sediments; sometimes the waters would be clear and sometimes more turbid, and shifting currents would ripple the surface.

At times the Peak District would have resembled an extensive fen-land of low-lying mudflats, swamps and sandbanks. Here primitive ferns would colonise the virgin territory followed by giant tree ferns and huge calamites only to be drowned and buried beneath fresh sediments eventually to form the Coal Measures on which Britain's industrial empire was built. The Coal Measures in the Peak District are, however, quite thin compared to the lowlands; the main outcrops occuring in the Goyt Valley near Derbyshire Bridge.

The alternating layers of sand, mud and grit became compressed to form a huge 'sandwich' seen notably in outcrop at Mam Tor. These sediments became the shales, siltstones and sandstones (Millstone Grit) of the Dark Peak. The presence of turbidites in these layers indicates the growing instability of the delta sediments followed by repeated subterranean landslides out into deeper water. This instability continues into the present as the thin layers are very loose and friable. When water seeps between them, catastrophic landslips result such as the 'Tower' at Alport Castles and Mam Tor itself.

The more gritty sands and gravels swept down from the northern mountains, were compressed and solidified to create the Millstone Grit or gritstone of the 'edges', Kinder Scout, Bleaklow and Black Hill. These coarse sandstones sometimes contain water-worn pebbles or grains of giant quartz or felspar which glint and sparkle in the sun and which belie their origin. Below many of the edges, such as Froggatt, Curbar and Baslow, piles of discarded millstones or grindstones can be found or even half-fashioned in the living rock. The coarse, abrasive nature of the rock made it ideally suited for use in the thousands of corn mills which once existed all over Britain. Later, Peak District grindstones were used to sharpen the products of Sheffield's famous cutlery and iron and steel industries. Synthetic carborundum grindstones eventually killed off the millstone market.

The 'edges' of the eastern side of the Peak are more pronounced and less folded and broken than those on the western side. Above these edges the gritstone is often eroded into exotically sculptured tors. These are the weathered remnants of harder rock which was rotted and weakened along its joints and planes underground, to be exposed and finally modelled by the wind, rain and frost in later ages. Great controversy still remains on exactly how this weathering occurred or indeed when, with some geologists favouring chemical weathering in the Tertiary while others supporting a more recent origin by frost-shattering during the last Ice Age. Nevertheless, powerful agencies have produced an extraordinary range of shapes and formations, like the Eagle Stone above Baslow Edge, the Salt Cellar and Cakes of Bread on Derwent Edge, and the Boxing Glove Stones and Woolpacks on Kinder.

Although the Millstone Grit forms the last layer of sedimentary rocks in the Peak District, it does not cover all of the Peak. The answer lies with a period of upfolding to form the Derbyshire Dome during the Variscan orogeny. During this folding, the western side of the Peak was severely crumpled into a series of narrow north-south folds and was subject to more fracturing and faulting than the eastern side. This great arch or anticline could have reached 3 040m at its highest point. Almost immediately, however, powerful forces of erosion set to work breaking down those heights by the combined action of frost, rain and running water. Because they were on top of the sandwich, it was the Coal Measures, shale and Millstone Grit in the centre which were worn away first, exposing the limestones underneath.

The present plan of the Peak District would have become established some 220 million years ago during the Permo-Triassic. Then the area would have been covered with the drifting sand-dunes of a desert much like the present Sahara. Then, perhaps for another 100 million years during the Jurassic, the Peak District became submerged by the waters of an inland sea. The age of the dinosaurs left little or no mark on the Peak District. The Tertiary is poorly represented and restricted to the deposits of silica sands at Friden, near Newhaven.

The last great destructive forces which shaped the Peak District were those of the glaciers during the Ice Age (Pleistocene). Not only did the glaciers have a direct impact through the deepening of river valleys and rounding of rocky summits, but glacial meltwaters following the Ice Age would have assisted in carving out the limestone caverns of the White Peak and deep gorges such as Winnats Pass near Castleton. Glaciers probably only totally covered the Peak District on two occasions during the 2 million years of the Pleistocene, in the glaciations known as the Anglian and the Wolstonian. During the last, Devensian, glaciation, the Peak experienced nothing more severe than a very cold, tundra-type climate, with extensive snowfields but no permanent ice cover. The ice brought with it vast quantities of ground-down debris, rocks and stones, picked up on its journey down from the highlands further north. This debris, in the form of a sticky, glutinous clay known as boulder clay (or till), can still be found in the Peak, together with 'erratics', rocks and stones carried far from their place of origin.

Peak District Links:

High Peak Net: The most extensive source of information on the Peak District

PeakLand

The Pennine Way

Ashbourne: Information on the gateway to the Peak District

The Peak District at Dawn (image)

Hathersage: Information on one of the Peak's famous mill towns

Bradwell: Many interesting links

Hope: Many interesting links

This page of geological notes was written by M J Lewis, Head of Geography (with Geology) at Northamptonshire Grammar School. The author has also supplied geological notes for Northamptonshire, Charnwood Forest and the Isle of Arran.