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GENERAL GEOLOGY

This section covers geology and biology.  This page introduces the main elements of the geology and mineralogy while biology will be found on a separate page.

structure of the Edge  |  faulting  |  mineralisation  |  minerals found at the Edge
protection of geology  |  geological visits  |  links

STRUCTURE OF THE EDGE

Alderley Edge is made of sandstones which dip from the Edge towards the Congleton Road (A34) at about 12? to the horizontal. The sandstone is not one solid mass but is in layers many metres thick. Each layer or 'bed' is slightly different in origin, hardness and colour. All the rocks at Alderley were laid down about 250 million years ago in the Triassic era.

The youngest rocks are the West Mine beds which are formed from air-borne sand and form the thickest layers at Alderley. Below them are the Wood Mine beds formed of alternating layers of conglomerate, water-deposited sandstone and clay (sometimes referred to as marl). Below these again are the hard beds of the Engine Vein sandstones which form the bottom of the Helsby (formerly Keuper) Sandstone Formation. It is these hard beds which outcrop (appear) at Stormy Point and provide the weather-resistant rocks that form the Castle Rock. Finally, appearing at the bottom of Stormy Point, are the soft rocks of the Wilmslow (formerly Bunter) Sandstone Formation.  All of these rocks are in the Sherwood Sandstone Group.

The table below shows the hierarchy of rock units (as per British Geological Survey with their designated acronyms: www.bgs.ac.uk and follow links to Lexicon).  There are further groups and formations outside the range shown.  The table shows the youngest rocks at the top.

SUPERGROUP GROUP FORMATION MEMBER DESCRIPTION PRESENCE AT ALDERLEY AGE
New Red Sandstone (NRS) Mercia Mudstone (MMG) Sidmouth Mudstone (?) Northwich Halite (NWH) Halite with partings of mudstone Not present Anisian
Bollin Mudstone (?) Bollin Mudstone Siltstones, mudstones and sandstones Not present on Edge but abutting to east Anisian
Tarporley Siltstone Formation (TSF) Tarporley Siltstone Siltstones, mudstones and sandstones Not present on Edge but abutting to east Anisian
Sherwood Sandstone Group (SSG) Helsby Sandstone (HSF) Nether Alderley Sandstone (NASM) Fluvial sandstone   Anisian
West Mine Sandstone (WMSM) Aeolian >> fluvial West Mine (aeolian)  Anisian
Wood Mine Conglomerate (WMCM) Sandstone with sporadic conglomerates, particular towards the base, and siltstones  Wood Mine (fluvial)  Anisian
Beacon Lodge Sandstone (?) Sandstone with sporadic conglomerates, particular towards the base, and siltstones    Anisian
Engine Vein Conglomerate (EVCM) Sandstone with sporadic conglomerates, particular towards the base, and siltstones  Engine Vein  Anisian
Wilmslow Sandstone (WSF) Sandstone with sporadic siltstones Lower part of Engine Vein Olenekian
Chester Pebblebeds (CPBF) Conglomerates Not present Induan

(Anisian period is middle Triassic: 237 to 246 million years BP, Olenekian and Induan are lower Triassic: 246 to 251 million years BP.  This table has been revised in 2016 with data from "The Story of Alderley", figure 5.8.)

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FAULTING

As well as the horizontal divisions in the rock formed by the beds of sandstone and clay, there are near-vertical divisions which were formed some tens or even hundreds of millions of years after the sandstone was laid down. These vertical breaks or 'faults' occur throughout the Edge and follow two predominant directions. The north-south faults have been responsible for most of the major movement of rock including the uplifting of the Edge itself and the steep hill on the Congleton Road . In fact, the whole area of land between the Congleton Road and the Edge has been raised several hundred metres as a block, called a horst, because of the faulting.

The other direction of faulting is roughly southwest-northeast and faults on this alignment are often mineralised. It is thought that the ores were carried in solution up through the faults, which are relatively porous compared with the surrounding clays and sands. Some lead is found in the faults (e.g.: at Engine Vein and at Stormy Point) but most of the copper and much of the lead is found dispersed in the sandstone on one or both sides of the faults.  The copper-bearing ore body is usually thickest near a fault and tapers away from the fault. Generally, ore is found only on the down-dip (south-western) side of the fault showing how it was deposited by water percolating through the sandstone after the land was uplifted. In West Mine, the association with faults is less clear-cut but this is probably explained by the fact that the rocks in West Mine are more porous and the solutions could carry further than in Wood Mine or Engine Vein.

One break with the general rule is the north-south faulting upon which the Cobalt Mine lies.  This fault can be traced from Findlow Wood to Saddlebole and it may have been mined for cobalt at several places along its length.

Click on images below for an enlargement.  Use your browser's back key to return to this page.

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Cross section of Alderley Edge. View from south showing the different sandstone, conglomerate and clay beds outcropping across the edge from west to east.  Extracted from Chris Carlon's books with changes and colours by NJD.

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Mineralised fault in Wood Mine. The fault is mineralised with galena and chalcocite. Close to the fault is the large stope known as North End Chambers where malachite, cerussite and manganese wad are found. The pink line picks out the fault plane.

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The outcrop at Stormy Point. The hard sandstones at the bottom of the Helsby formation outcrop at Stormy Point and form the abrupt edge.

Vein in Engine Vein

The "vein" in Engine Vein.  The picture shows the fault (mid-right of photo) under the concreted area.  The red lens entering from the left is a clay band.

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MINERALISATION

The mineralisation of Alderley Edge has been a matter of speculation for centuries.  We do not know what ideas the Bronze Age or Roman miners had about the deposition and disposition of minerals but it is fair to assume that they would recognise the link between minerals and faults, even if they had no idea what a fault was.  Certainly the mediaeval miners would have recognised that there were veins at Alderley, resembling the veins in Derbyshire. In the nineteenth century, geology was developing as a science and geologists were drawing up ideas about the deposition at Alderley. Early theories, based on the malachite deposits being worked at the time, said that the ores had been deposited "syngenetically", in other words at the same time as the sandstone.  This theory was based on the way in which ore was found and extracted in "beds".  However, by the twentieth century, most geologists accepted that the ore was laid down in the faults, after the sandstone was deposited.  This theory, referred to as the "epigenetic" method, was much closer to the truth as we understand it now.

In brief, the present idea is that the deposits are not hydrothermal (i.e. from a magma source) as the chemistry does not match that from hydrothermal sources.  Recent research work looked at sulphur isotopes and inclusions in the rock and deduced that the mineral had been deposited from cool solutions.  The process by which the ores reached Alderley as malachite and azurite dispersions is considered in four stages: (a) origin, (b) transport, (c) precipitation and (d) redeposition.

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SOME OF THE MINERALS FOUND AT ALDERLEY EDGE

Malachite

Green copper carbonate, extensive

Azurite

Blue copper carbonate, frequent

Chalcopyrite

Copper iron sulphide, on veins

Bornite

Similar to chalcopyrite

Cuprite>

Copper oxide

Chalcocite

Copper sulphide>

Wulfenite

Lead and tungsten, yellow crystals

Chrysocolla

Blue green deposit on surfaces

Barite

Barium sulphate, close to veins

Galena

Silvery grey, lead sulphide, frequent

Cerussite

White lead carbonate

Pyromorphite

Lead chloro-phosphate, apple green

Wad

Black manganese oxide

Asbolite

Wad containing over 20% cobalt

Mottramite

A lead mineral containing vanadium

Silica

Sand and quartz, the main country rock

For more detail, refer to the page on mineralisation.  

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PROTECTION OF THE GEOLOGY

Alderley Edge is protected by being a Site of Special Scientific Interest (SSSI, often called a triple-SI) administered by Natural England.  The site is reference number 1001677.  Details of the site can be found on the Natural England Database.  More about SSSIs is provided n the DCC Legal Issues page.

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VISITS

We are able to offer geologically themed visits to the mines. To find out more, go to the geological visits page.

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LINKS

Another website produced independently of the DCC provides more detail of the geology.  This other website has not been vetted by the DCC but may be of interest to readers: www.alderleyedge.org/Mines/geology.htm [page currently unavailable but accessible at http://homepage.ntlworld.com/c.batty10/edge/Mines/geology.htm].  There is also a Wikipedia page about the geology at http://en.wikipedia.org/wiki/Geology_of_Alderley_Edge.  More references can be found by searching the internet.

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Last updated: 30/01/2016
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