A Taste of the Peak District

Accommodation, activities and attractions of the Peak District of the UK

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The Peak District was the first region in the UK to be designated as a National Park. Read a little about the geology of the area on this page in chronological order. The rocks are mainly Carboniferous in age and include examples of coal measures, Millstone Grit, Yoredale series and Carboniferous limestone groups. There are also basaltic lava flows and a vent agglomerate. Overall the thickness of the Carboniferous in the Peak is about 3000m.

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Click here for the Open University Geology East Midlands branch web site.

Basement rocks of the Peak District

The basement rocks are not exposed anywhere in the Peak District but have been located in boreholes. There are 3 of these, Eyam, Woo Dale and Caldon Low. The Eyam borehole hit Ordovician rocks at a depth of about 1800 metres depth, Woo Dale near Buxton hit volcanic rocks of an uncertain age at about 270 metres and Caldon Low entered rocks of probable Devonian age at a depth of about 350 metres.

Early Carboniferous rocks of the Peak District

The Peak District was similar to how the Bahamas banks are today with a sea floor of varying depth with shelf areas. This was created by movements in the northern shelf of the E-W land mass, St. George’s Land. Mud mounds formed on the shelf areas, now seen as reef knolls. At the edges of the shelf, apron reefs formed, as seen at Castleton. The dip of the Castleton apron reefs is an actual dip, evidenced by the partial infilling of brachiopod fossils (geopetal infill). This type of infill acts like a spirit level so what we are seeing here is the sea floor, as it was at the time of deposition. In the deeper off-shelf regions, the limestones are darker. This is because their deposition was accompanied by the deposition of mud as well. Limestone is principally calcium carbonate and is formed in different ways. Either from the incorporation of shelly material into the sediments or more commonly by biochemical precipitation by algae or other zooplankton. Limestone that has no discernible fossils is called limestone or sometimes micrite; if it has a range of obvious fossils it is termed a fossiliferous limestone; if it is composed mainly of shell fragments it is called a coquina; if it is mainly the exoskeletons of microscopic organisms it is chalk and if it is made up from small sand grain sized spheres of calcite it is an oolitic limestone. The latter two types are not seen in the Peak District. An altered type of limestone, dolomite is considered in the mineralisation section. In many places, chert nodules (lumps) have formed within the limestone for example at Intake quarry near Middleton top on the High Peak Trail. It is more resistant to weathering and at Intake, provides climbers with interesting holds. Chert is a form of quartz (silica - silicon dioxide) with no crystalline structure. It is formed where diatoms and radiolarians (microscopic creatures) have died and been concentrated as a siliceous ooze. These organisms are unusual in that they can extract the tiny concentrations of silica dissolved in seawater. The equivalent in chalk is flint, which has different properties, making it suitable for making tools in the sone ages. The varied topography of the region is best explained by crustal stretching thinning as a result of plate tectonics. Near Calton Hill, mantle material has been identified in a volcanic vent. This is characteristic of areas where the crust is under tension and has been thinned.

Later Carboniferous rocks of the Peak District

The sea level fell in relation to the land so some rocks were eroded. In deeper areas deposition continued without a break. Either side of the Upper and Lower Carboniferous boundary, which is marked by a marine band with goniatites, shales (clayrocks) and impure limestones can be observed. Later, flooding brought a resumption of sedimentation over most of the Peak District but the rocks were of a very different nature to before. No more limestones are seen, instead there were muds and sand. The clouding up of the water made it hard for marine life to survive. This change was caused by the encroachment of a vast delta to the north of St. George’e Land. This delta was the source of the Upper Carboniferous gritstone, useful for making millstones and hence named millstone grit sometimes. Gritstone is a well cemented coarse sandstone and provides climbers with a superbly varied rock upon which to climb. The cement is primarily silica, making the rock resistant to erosion, this indicates that deposition was in acidic waters. There are local pockets of calcite cementation, indicating localised pockets of alkalinity - these are often associated with bacteria that decay plant materials and could be sites of log jams on the braided streams of the delta. These erode more easily that the silica cemented bulk of the grit and have created caves at many of the areas gritstone crags like Robin Hood’s cavd at Stanage Edge, keyhole cave at Millstone quarry and the cave at Froggatt edge. Derbyshire at this time was still well south of its location today and the top of the delta was colonised by lush vegetation. The warm swamps formed thick layers of peat which became the coal of the Derbyshire, Notts and Yorkshire coalfield. The delta was occasionally flooded by the sea which gave rise to thin fossiliferous (goniatites) marine bands. Since these are quite thin, they were short lived and one explanation that has been suggested is the melting of polar ice caps.

Post Carboniferous

Little can be discovered about the geology of the Peak District after the end of the Carboniferous as any rocks that were formed have been completely removed. It is possible that the region was covered with Permo-Triassic deposits as there are outcrops of this age that lie close to the boundaries of the area e.g. the Churnet valley. These deposits may have infilled the late Carboniferous topography and were removed in an ice age. There is evidence that a lot of the current topography are re-excavated earlier features, e.g Castleton, Chrome Hill, Thorpe Cloud, the wind sculpted shapes in the dark Peak, created when conditions were very different to today. The dolomitisation of the limestones at Harboro Rocks has not been satisfactorily explained. The main area in the UK for dolomitic limestone is, guess what, the Permo-Triassic (NE England) so it is possible that the limestones at Harboro were altered as a rsult of Permo-Triassic conditions. Now there’s an idea for your postgrad studies! Please let me know what you find out.Other evidence that the Peak District had a covering of Permo-Triassic rocks comes from the presence of clays in the Trent valley, derived from Permo-Triassic marls. This is a very soft rock and could easily have been scraped to the valley by the action of an ice sheet. Also some of the Peak District rocks show a significant reddening e.g. the Roaches. Rocks that are weathered under arid conditions tend to be reddened. This would fit in with the bigger picture of the formation of the supercontinent of Pangaea. As the sea to the north of St George’s Land silted up because of the Upper Carboniferous delta and the seas to the south were destroyed by plate tectonic action, it became landlocked. Much closer to the equator than now, arid conditions would have resulted. Also, the crustal movements changed from tensional to compressional, creating the folds of the Derbyshire Dome. There is no evidence whatsoever to indicate if Cretaceous or Jurassic sediments were ever deposited here but deposits from the Miocene-Pliocene boundary have been found (and exploited) in the Brassington region. Rivers deposited sands into pre-existing solution and subsidence hollows. The Peak District was at the very southern limit of the last glaciation (the Devensian) and this has left behind glacial drift in the form of boulder clay and meltwater sands and gravels. The most recent deposits are the soils, acidic over the gritstone areas and alkaline over the limestones. The peat of the Dark Peak may have been triggered by the large scale clearance of the forests during the bronze age.

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