GEOLOGY

Leader: Derek Brumhead  

Visit to Alderley Edge.

9 April 2006

[Click on images to enlarge]

Introduction

Alderley Edge is an escarpment of sandy and pebbly rocks of Triassic age (248m -208m years ago) rising to a maximum height of 80m above the plain to the north.  The escarpment is capped by the Helsby Sandstone Formation (formerly known as the Lower Keuper Sandstone) a very pebbly sandstone with barite cement. It rests on the Wilmslow Sandstone Formation (formerly known as the Bunter Upper Mottled Sandstone) over 300m thick forming the steep slope down to the Mottram road. The conglomerates at Alderley Edge form part of sequence of conglomerates, sandstones and marls deposited in a desert environment by braided shallow rivers periodically flooding. The non-pebbly Wilmslow Sandstone appears to be part water-lain and part wind-blown sands. Fossil remains are rare, the most well known being Euestheria a small crustacean found in a mudstone bed, and the three-toed footprints of a dog-sized running dinosaur, Chirotherium, found elsewhere in Cheshire.

The general dip of the rocks is towards the south-west at about 10-15 degrees. forming the southern limb of the Wilmslow Anticline . The Edge, is formed from the southern limb of the eroded anticline and is made prominent because of the resistant nature of the conglomerates and barite-cemented sandstones.

The rocks at Alderley Edge have been broken into small stepped blocks by many small NW-SE faults; the scarp is repeated several times, the faults throwing to the north-east. Two large faults terminate the Edge to the east and west. These have lifted up the Edge into a single large block or 'horst', two miles in length.

Mineralisation at Alderley Edge is locally concentrated in the vicinity of these faults. A wide variety of minerals, particularly copper, have been found at Alderley Edge. They form uneconomic deposits now but at several periods have been worked at a considerable profit. Mining goes back to Bronze Age times.

Park at the National Trust car park and then followed the itinerary shown in the map.

1. Engine Vein

This mine is a deep gully following an NW-SE fault with a downthrow of 12 feet to the north. One rock face shows scratches (slickensiding) resulting from the movement, and there is a mudstone displaced on the upthrow side of the fault (south). The fault provided a passage way for the primary migrating mineralising fluids to rise up from depth, from where they were redeposited in the porous sandstones and pebbly rocks. Flecks of the copper ore malachite (green) and the characteristic, but rare, tiny blue spheres of azurite can be found with diligent searching.

Engine Vein

The rocks here are pebbly sandstones (the Engine Vein Conglomerate), deposited in a desert plain, a high energy environment, as point bars in shifting river channels. Since the beds migrated as the rivers changed their course, cross bedding is typical. Mudstone beds were also deposited by overbank floods forming pools or lakes.

Slickensides on the fault plane in the Engine Vein

2. Old Alderley Quarry

This interesting abandoned quarry shows several geological features of note. Higher in the succession, the rock is a sandstone, fairly free of pebbles and has been quarried to be used as building stone in the Alderley Edge area. A small fault shows a marl (mudstone) bed downthrown to the north which has been followed by the roots of a tree. The fossil Euestheria ( a shrimp) has been found here. The face of the rock is covered in pick marks, where the quarrymen have followed a line of weakness or joint to free up large blocks of sandstone. This quarry is a good location to demonstrate true dip and apparent dip, for looking along the strike the bedding of the rock shows a true dip of about ten degrees, but on another face at right angles it looks horizontal.

Old Alderley Quarry showing marl band

Old Alderley Quarry showing pick marks

3. Open Cast

This is another mine opened out along a fault line.

Stormy Point

Conglomerate. Wind-faceted pebbles may be found

4. Stormy Point

Here, the basal beds of Engine Vein Conglomerate sit on a distinctive mudstone bed. The pebbles are mainly quarzites and vein quartz with a variety of basic igneous rocks. At the time of deposition they were eroded from mountains sited where the Midlands are today and washed northwards into the desert plains. Rare wind-faceted pebbles, ventifacts as they are called, can be found. The top surface shows veins of barite, standing proud, rather like tree roots. The barite also forms a distinctive cement between the sand grains, showing as desert rose 'ghost' crystals. In good visibility there are splendid views from here over to Kinder.

5. Doc Mine

'Devoid of Copper' has been suggested! The sides of this mine entrance show the sandy strata dipping south followed by the floor of the mine. The impregnation of the sand grains by the copper mineralisation (a secondary deposit) is well shown here at the mouth of the mine.

Doc Mine

Sandstone impregnated with malachite at entrance to Doc Mine

6. Pillar Mine

Named after an isolated pillar of sandstone which was destroyed some decades ago. The mineralisation includes a black mineral, asbolite, a cobalt-nickel ore. The rock here is the Wilmslow Sandstone Formation, beds of variegated coloured fine sand. It lies stratigraphically under the Engine Vein Conglomerate, which here has been dropped down on one side to the same level by a landslip.

7. Castle Rock

Engine Vein Conglomerate standing above the softer Wilmslow Sandstone. Here you are standing on the dip slope of an anticline, which rises towards the north. Pete Webb, who is a consultant geologist (former school student of Derek's!), described how the bedding could be distinguished from water deposited sediments. Examine the Wilmslow Sandstone in the cliff below, where the junction with the Engine Vein Conglomerate is well seen. These sands are aeolian deposits (wind blown).

Castle Rock

Barite vein in sandstone

Helsby Sandstone Formation [formerly Lower Keuper Sandstone] resting on Wilmslow Sandstone Formation [formerly Bunter Upper Mottled Sandstone] below Castle Rock

Junction of Helsby Sandstone Formation (above) and Wilmslow Sandstone Formation (below)

Mining Museum

The visit can end most appropriately with a visit to the mining museum next to the car park, which is only open on Sundays. Here, the enthusiastic and knowledgeable members of the Derbyshire Caving Club describe the occurence and mining of the minerals and there is a good display of all the artefacts they have found underground over the last twenty five years. There is also an old mine shaft inside the museum from which the mineral asbolite was mined.

References:

• D B Thompson, 'Triassic Rocks of the Cheshire Basin: Alderley Edge I and II' in R M C Eagar and F M Broadhurst, Geology of the Manchester Region, Geologists' Association Guide No 7, 1991.
• C J Carlon, The Alderley Edge Mines, Altrincham 1979.
• S Timberlake and A J N W Prag, The Archaeology of Alderley Edge, Manchester 2006.