Geology of Alderley Edge

Infobox Mountain
Name = Alderley Edge
Photo = Stormy Point 1993.jpg
Caption = Stormy Point
Elevation = 183 m (600 ft)
Location = Cheshire, ENG
Range =
Prominence =
Coordinates =
Topographic
OS "Landranger" 118
Grid_ref_UK = SJ870760
Grid_ref_Ireland =
Listing =
Translation =
Language = [|Pronunciation =

Alderley Edge in Cheshire is one of the classic locations for the study of Triassic sandstones in the United Kingdom. Numerous scientists from the early 1800s up to the present day have studied the area and it is a popular field site for universities around the UK. As time progresses, new theories are developed and new hypotheses are tested as to the origin of the sandstones. The information presented here provides the history and current (as of 2006) thoughts on the geological evolution of the sandstones at Alderley Edge.

Nomenclature

The nomenclature of the English Triassic was rewritten in the 1980s [ [http://www.jncc.gov.uk/pdf/V24Chap1.pdf Permian and Triassic Stratigraphy] ] and many of the previous names were changed. The classic terms ‘Bunter’ and ‘Keuper’ have now been abandoned, and the formations now recognized in the UK Triassic sequences, constitute three major lithostratigraphical units,

*the Sherwood Sandstone Group (arenaceous siliciclastic) (equivalent to the Bunter and lower Keuper)
*the Mercia Mudstone Group (argillaceous,halitic) (equivalent to the Keuper Marl)
*the Penarth Group (not seen at Alderley). [cite journal
last = Ruffell
first = A
coauthors = Shelton, R
title =
journal = Journal of the Geological Society
issue =
pages =
date = July 1999
id =
accessdate =]

For this reason, in places, the older names with appropriate cross references are used, in order to maintain an understanding of earlier work.

"For a description of some of the other terms used see Geological unit"

History

The geology of Alderley Edge has fascinated people from all walks of life, scientists, miners and tourists for hundreds of years. Bakewell [cite journal| last = Bakewell| first = Robert| coauthors =| title = Letter to the Magazine| journal = Monthly Magazine| issue = February | pages =| date = 1811| id =| accessdate =] describes it thus:

"The hill is evidently of alluvial formation, being composed chiefly of gravel and soft white and reddish sandstone, - the white is intermixed with rounded quartz [pebbles, the red with particle of mica. In some parts the red and white sandstone assume a nearly stratified appearance, in others the red stone intersects the white in very thin seams, branching in various directions. In the white sandstone are found various ores of lead as small portions of galena and in the same granular state intermixed with sandstone. In other places particles of blue and brown were collected in nodules of various sizes and imbedded along with pebbles in the sand rock like currants in a pudding".

A few years later, Ormerod in his book the ‘History of Cheshire’ [cite book
last = Ormerod
first = George
coauthors =
title = The History of Cheshire
publisher = Routledge and Sons, Ludgate Hill, London
date = 1882 (2nd Ed)
location =
pages =
id =] describes it as follows:

"Alderley Edge is an abrupt and elevated ridge, formerly the site of a beacon, which bears the appearance of having been detached by some great convulsion of nature from the range of the Macclesfield hills. Near the summit cobalt ore, lead and copper have been got in small quantities. The sides are varied with cultivated land, wood and rock; and the entire mass presents a striking object to all the surrounding district over which it commands a most extensive prospect".

Regional Setting

The Cheshire Basin is today one of the classic onshore localities in the UK [ [http://www.english-nature.org.uk/special/geological/sites/area_ID5.asp English Nature] ] for the study of Triassic red-bed fluvial and aeolian sandstone sequences, and provides important insights to the nature and evolution of deformation in continental clastic natural gas and petroleum reservoirs, such as those of the Rotliegendes in the Southern North Sea Gas Basin and of the Sherwood Sandstone in the adjacent East Irish Sea Basin. Analogues for cemented cataclastic faults, which can compartmentalise reservoirs, are well displayed by the arrays of deformation bands within the Alderley outcrops.

The Cheshire Basin

Regionally, the Cheshire basin is a deep fault bounded half-graben containing up to 4.5 km of Permo-Triassic sediments. The basin is approximately 100 km North - South and up to 50 km east-west and is bounded on the eastern margin by the Wem-Red Rock fault system. Syn-depositional movements on the Wem-Red Rock fault system during the Triassic had a significant effect on the subsidence and sedimentation rates within the basin. Crustal uplift and erosion during the Tertiary were a direct result of reactivation of these earlier faults. Today active movement of the Wem-Red Rock fault system causes minor earthquakes across an area from North Wales to Lancaster. An interactive earthquake map can be found at the British Geological Survey Site. [ [http://www.earthquakes.bgs.ac.uk/recent_events/uk_special/alert_info_uk.htm Interactive earthquake map] ]

The sediment infill of the basin is of Permian to lower Jurassic age and is up to 4.5 km thick in the south-east suggesting that greater and later subsidence occurred in the basin than in any other British Permo Triassic basin. [Smith N.T. The Application of Seismic Sequence Stratigraphy to the Cheshire Basin: Implications for Permo-Triassic Basin Development [http://www.bdrg.esci.keele.ac.uk/smith.htm Keele University] ]

An extensional regime in the early Permian is believed to have initiated major faults which were to form the basin margins, the most notable of these is the Wem-Red Rock fault system in the east. Subsidence related to this fault movement heralded the beginning of basin formation and of infilling by early Permian sediments, the distribution of which is thought to have been controlled by the NE-SW faults parallel to the Wem-Red Rock faults. Basal Permian breccias deposited as alluvial fans lie unconformably. Manchester Marls represent a brief transgression into marine conditions in the mid Permian, their distribution and increased thickness of Permian sediments in the north indicates a depositional centre towards the north or northeast of the basin at this time. Basin subsidence was initiated in the early Triassic when subsidence related to movements on the faults on the eastern margins of the Worcester and Cheshire basins led to a connection between them and to the evolution of a drainage system with rivers sourced in the Variscan foreland.

The aeolian-fluvial sandstone formation represents the uppermost unit of accumulation within the Sherwood Sandstone Group of the Cheshire Basin (Mid-Triassic). Three distinct members (Thurstaston, Delamere and Frodsham) are recognised on the basis of lithofacies type. Stratigraphic correlation of the formation within the northern and central parts of the Cheshire Basin has been undertaken using a combination of outcrop, core, well-log and historical data to provide regional-scale, district-scale and high-resolution local-scale overviews of the stratigraphic architecture. At the regional scale a 30-km-long correlation panel has been constructed using data from 17 key localities along a transect from Runcorn to Delamere Forest at the southern end of the basin. Variations in the thickness of the formation between several fault-bounded blocks may reflect syn-tectonic sedimentary evolution of the succession. Cycles of fluvial-to-aeolian sedimentation, possibly driven by periodic climatic fluctuation are easily recognised within individual fault blocks but can't be traced continuously across the entire region. A 10-km-long correlation of the Delamere Member within the Runcorn-Frodsham (15km to the west of Alderley) member shows a laterally extensive but thin (1 to 7 m thick) series of mudstone bands with associated aeolian cross-bedded sets that punctuate the predominantly fluvial strata. These are interpreted as the uppermost part of a drying-upward cycle that represents the fluvial system giving way to aeolian activity prior to the onset of renewed fluvial flash flood activity.

The detailed stratigraphy of a 30-metre-thick interval of the central part of the Delamere Member has been examined in a series of quarries at Runcorn Hill. A 400-metre-long quantitative panel was examined and it documents the stratigraphy of a mixed fluvial and aeolian succession. Here, the aeolian is interpreted as the deposits of sinuous-crested transverse barchanoid dunes that exhibited obliquely migrating scour-pits on their lee (downwind) slopes. [cite book
last = Hull
first = E
coauthors = Green A H
title = The Geology of the Country around Stockport, Macclesfield, Congleton and Leek
title = Memoir of the Geological Survey GB
publisher =
date = 1866
location =
pages =
id =]

Upward fining facies successions within the Members are characterised by planar laminated, ripple-bedded and cross-bedded sandstones with abundant intraformational rip-up clasts, exotic clasts and water escape structures. These successions are interpreted to represent waning flood conditions in an ephemeral fluvial system conditioned by semi-arid climatic conditions. In the Runcorn district of the basin one 20-m-thick fining-up succession culminates in a series of thin (5-20 cm) red sandy-mudstone beds that are interbedded with sets of cross-bedded soft sandstones characterised by grain flow and wind-ripple laminae indicative of aeolian dune accumulation. The sandy-mudstone beds including the Alderley beds are characterised by wavy laminae, desiccation cracks, mud flakes, raindrop imprints, loads and flutes.

This sedimentological evidence, together with the association of the mudstone beds with the aeolian dune units favours an interpretation of fluvially flooded interdune ponds that were periodically subject to desiccation and re-flooding. In places the mudstone beds pass laterally into horizontally laminated wind-ripple beds that indicate a transition from wet, through damp to dry interdune conditions. These interdune strata exhibit a ‘feathered’ relationship with the toe-sets of the overlying aeolian dune units, signifying dune migration that was contemporaneous with damp/wet surface conditions within the adjacent interdunes. This indicates that the aeolian dune-interdune system underwent ‘wet-climbing’ whereby accumulation of both dune and interdune strata continued uninterrupted through minor flooding events.

Red Bed Deposits

The Alderley sandstones are described as classic Red Bed deposits. 'Red Beds’ are a distinctive sedimentary facies traditionally associated with non-marine depositional environments such as alluvial floodplains and arid deserts. They range in age from Early Proterozoic to Cenozoic and are regarded as geochemical indicators of oxidizing conditions. In the past there has been much debate whether these continental red beds are primary (formed during deposition) or diagenetic (post-depositional) in origin. [cite journal
last = Mohr
first = P A.,
coauthors =
title = On the Copper Mineraised Sandstones of Alderley Edge, England and Chercher, Ethiopia and the Problem of their Genesis: an essay on red bed copper deposits
journal = Geophysical Faculty of Science, Haile Selassie University, Addis Ababa
issue = Ser A,.
pages = pp4
date = 1964
id =
accessdate =]

Red Beds are coloured by finely disseminated ferric oxides, usually in the form of hematite (Fe₂) O₃) although a range of other iron oxyhydroxide minerals are normally present. Oxidation-reduction (Eh) and pH control hematite formation and the observed minor colour variations often seen in red beds. In general, the colour variations relate to changes in pore water chemistry resulting from fluctuations in the depositional or early diagenetic environment. In particular, intercalated marine units are normally non-red because the high levels of organic productivity produce reducing conditions below the sediment-water interface. [cite journal
last = Van Houten
first = F. B.,
coauthors =
title = Origin of red beds. A review -1961-1972
journal = Annual Review Earth Planetary Science
issue = 1
pages = pp 39–61.
date = 1973
id =
accessdate =]

Triassic in northern England

The Triassic succession of Great Britain is divided into a number of stratigraphical units that are generally hard to assign to divisions of the Lower, Middle, and Upper Triassic. [cite journal
last = Warrington
first = G
coauthors = Audley-Charles, M.G., Elliot, R.E., Evans, W.B., Ivimey-Cook, H.C., Kent, P.E., Robinson, P.L., Shotton, F.W. and Taylor, F.M.
title = A correlation of Triassic rocks in the British Isles
journal = Special Reports of the Geological Society of London
issue = 13
pages =
date = 1980
id =
accessdate =]

The upper boundary of the Triassic is defined by the base of the Jurassic System, [ [http://www.jncc.gov.uk/earthheritage/gcrdb/GCRblock.asp?block=70 Joint Nature Conservation Committee (UK Government Department)] ] which is internationally recognized as being at the base of the planorbis subzone of the planorbis Ammonite Zone of the Hettangian Stage.

The Triassic Period (251Ma – 205Ma) is part of the Mesozoic Era (251Ma – 65Ma) and is so called from its former threefold division in its type locality in Germany. The divisions have been revised in nomenclature so that they do not correspond with most of the older literature. The onshore Triassic in Britain differs from that in the southern North Sea, Germany and other parts of northern Europe, in not being the classic tripartite lithostratigraphic subdivision; The Triassic deposits of Germany form three series. In the Bunter (meaning 'brightly-coloured') series, the land was emergent and red sandstone and sandy shales, with some salt and gypsum, were deposited. The Muschelkalk series saw the transgression of the land by the sea and the deposition of marine shale and limestone; the Keuper series saw the land again emergent and shale, sandstone, and gypsum being formed. Instead, the British Triassic is divided into the Sherwood Sandstone Group, the overlying Mercia Mudstone Group (previously described by their quasi Germanic names, the Bunter and the Keuper) and the Penarth Group [ [http://www.geography.lancs.ac.uk/cemp/publications/trft-intro.pdf University of Lancaster] ] these together attain a maximum thickness of ca. 3.5 km in the Cheshire Basin.. In Britain the middle division of marine beds is largely absent, giving the original twofold division into Keuper and Bunter, which whilst recognizable as lithological or facies divisions, they are not equivalent to the strata in Germany. The absence of the middle facies, the Muschelkalk, does not mean that Muschelkalk beds of the middle Triassic are absent, but rather that the deposition was continental over this area and not marine as over northern and central Europe. Dating is therefore difficult as there are no fossils and there was no igneous activity, which could have lead to the emplacement of isotopes enabling dating. Recent work has shown that the Mercia Mudstones are more complex than originally thought and there are now proposals to give new stratigraphic names to four of the formations. [cite journal
last = Wilson
first = A. A.
title = The Mercia Mudstone Group (Trias) of the Cheshire Basin
journal = Proceedings of the Yorkshire Geological Society (Proc. Yorks. Geol. Soc.)
volume = 49
issue = no3
pages = pp. 171–188
date = 1993
id = ISSN 0044-0604 ]

Permo – Triassic

By the Triassic, the Permian Zechstein Sea had retreated and the climate had become a little wetter giving a gentle transition making the Permian-Triassic boundary uncertain in northern England as there are no fossil horizons or facies changes that make a definitive separation possible as there in continental Europe. The horizon however is characterised by a succession of red marls (calcareous mudstones) deposited on coastal flats, followed by the Sherwood Sandstone (formerly Bunter Sandstone). The 'British Isles' were not islands, but had an intra-continental position within Pangea. The area that now constitutes Great Britain was drifting northwards as Pangea rotated, was at a latitude of 10° - 20° north, equivalent to the latitude of the present day Sahara desert. Erosion of the then recently uplifted landmass formed Aeolian deposits in the southern and central parts of the country. This iron-rich silica sandstone was both oxidised and reworked to give it its red colouration and its name, "New Red Sandstone". British deposits of the era consist of these red beds, alluvial, fluvial and lake deposits, with some shallow-water marine and evaporite deposits. These Permo-Triassic outcrops can be seen on either side of the Pennines and in Devon. Within the main central England basin (Staffordshire and Cheshire), the deposits are dominated by pebbly sandstones and conglomerates (Chester Pebble Beds, Wilmslow and Wildmoor Sandstones), which have been interpreted as the deposits of a fluvial system running within well-confined channels. The Sherwood Sandstone Group comprises a series of conglomerates, coarse sandstones and mudstones. The Chester pebble beds to the south of Alderley represent material deposited in alluvial fan/braided river system. [cite book
last = Carlon| first = Chris| coauthors =| title = Alderley Edge Mines| publisher = Sherratt, Manchester| date = 1979|location =
pages = 144 pages | id = ISBN 085427053X] The finer sediments of the Wilmslow and Helsby Sandstone to the west of Alderley represent alluvial deposits of low sinuosity channels. The Alderley area represents the mid point between the full braided river system and the lower energy area. Minor aeolian dunes and channel infill deposits in the Wilmslow Sandstones indicate an inter channel area or seasonal drying of some of the minor river channels.

On a more global level, to the east of what is now England, the new Tethys Ocean, extended south east through the Mediterranean region then east through the Middle East to the Himalayas and to India. During the Triassic a subduction complex including an elongate volcanic arc system developed along what would become the North American east coast. North Africa and Europe were still attached to North America as part of the supercontinent, Pangaea.

The Lower Triassic

The Sherwood Sandstone Group can be broadly divided into an upper (previously Keuper) and a lower (previously Bunter) unit, at the level of a widely recognized intra-Sherwood Sandstone disconformity perhaps within the uppermost Lower Triassic This disconformity separates two quite distinct environmental systems. This disconformity is widely assumed to be equivalent to the Hardegsen disconformity of the central European/Southern North Sea Basin, although the age of this disconformity is not constrained as it is in Europe by biostratigraphical indicators

Below the intra-Sherwood Sandstone disconformity these deposits are dominated by evidence of a major braided river system, this river system responsible for transporting the sand and gravel to the Alderley area was named by Wills (1970) the Budleighensis River, after Budleigh Salterton in Devon, where its existence was first established.

Within the western onshore basins there are several clues indicating that the river flowed in a northerly direction. Within the Alderley conglomerates, there are well rounded liver coloured quartzite pebbles from a source found only in the Variscan mountains of Brittany, France. Some pebbles contain microfossils of marine animals (Lingula lesueuri [cite journal
last = Alvaro| first = Jose Javier| coauthors = Daniel Vizcaino| title = Controverses geologiques sur le Cambro-Ordovicien de la Montagne Noire (France)| journal = Geodiveritas| issue = 24(4)| pages = | date = 2002| id =| accessdate = ] and Orthis budleighensis) which can be traced to the same source area in the Amorican Massif.

Pebbles of Carboniferous limestone and reworked earlier Triassic sandstones indicate their source as being from the South Derbyshire area and the Clent Area south of Birmingham respectively. There is further evidence from the distribution of the pebbles themselves, the average size of the stones on the southern margin of the basin is noticeably larger than that to the north, which is consistent with a flow of water from the south.

The river system flowed northwards through the Wessex Basin, the Worcester Basin, the various midland basins and on into the East Irish Sea Basin just to the north west of the Alderley area. A second branch probably flowed eastwards into the southern North Sea. This Budleighensis river system is evident by a series of sandstones with generally northwards directed palaeocurrents, which can be traced along a south to north line along the central parts of Britain.

Indications are that it probably had large seasonal changes in discharge, evident by cross-bedded sandstones deposited at stages of lower flow, although whilst the flow was seasonal, it is perhaps doubtful if this system was ephemeral in nature for there is relatively little evidence of any large scale aeolian sandstones in the basinal settings of the Midlands. It is clear that more locally sourced material such as carboniferous limestone and reworked earlier Triassic sandstone from the Clent area just south of Birmingham were also important components of the river systems as they flowed northwards giving some indication of the relief of the basin margins.

Towards the end of the Triassic, the sea level once again rose and periodic flooding caused by high spring tides and strong on-shore winds led to the formation of on shore saline lagoons or sabkha environments. [ [http://www.northeast-geolsoc.50megs.com/pages/page13permiantriassic.htm North East Geological Society] ] (A sabkha is a wide area of coastal flats bordering the sea, the name coming from certain coastal areas of Arabia). Intense evaporation from these lagoons resulted in the precipitation of a carbonate-sulphate complex and the thick halite beds as seen to the south west of Alderley in Northwich where the salt is mined commercially. It was in this type of environment that the Mercia Mudstone Group (formerly Keuper Marl) was deposited.

The sequence of formations in the Sherwood and Mercia mudstone groups in this region illustrates clearly the upward transition from continental fluvial to deltaic and littoral marine and ultimately to the hyperslaine epeiric sea environment of the Mercia Mudstone group.

The Alderley Sandstones

The upper division (ex-Keuper) follows the lower (ex-Bunter) unconformably. The upper is very thick and in Cheshire reaches a thickness of about 1250 m, locally conglomeritic, but comprises mainly of fine water lain sandstones.

Interest in the area took off with vigour in the mid 1800s with the first real efforts made to quantify the strata of the country using the new mapping methods that geologists like William Smith had pioneered. The economical value of the area in those times lead to a detailed examination of Alderley. Boreholes starting to be used for geological logging and research whereas previously they had been used simply for wells with no real interest taken in the actual geological make up of the rock below. In 1894, a borehole was sunk to 280ft below surface and the strata recorded in a scientific manner. This borehole, the Isaac Massey (NGR SK 84237819) was the first serious attempt to understand the geology of the area. The area has since been subject of much more detailed research and still provides many clues to basin formation and mineralization processes. The original borehole was crude by today’s standards but it gave an overview of the strata below the Alderley area.

Now the Wilmslow Formation, (part of the Sherwood Sandstone Group), the lower Triassic was previously a threefold division, the Lower Mottled Sandstone, The Pebble Beds and the Upper Mottled Sandstone, (only the later is seen at Alderley) which merged into the Conglomerates of the upper Triassic. The Upper Mottled Sandstone now the lower Wilmslow Member is a medium to coarse-grained friable false-bedded sandstone with abrupt colour changes from bright red to white. It is composed mainly of rounded grains but sub-angular grains also occur. Normally the rock lacks coherence and weathers into sand down to 6 m and even unweathered rock can be crushed to sand easily. However near faults, ghost crystals of barites make the sandstone harder and resistant.

Lower sandstones

The lower sandstone (Upper Mottled Sandstone) has a full thickness of 305 m generally a reddy brown colour with some white patches, which give it its name, these patches are often associated with organic nuclei. The sandstone has two distinct types, a bright dark red known as the Moulding Sand and mottled paler sandstone. The Moulding Sand is so called after its use in the foundry industry for making the casting moulds. Resting on top is a conglomerate, which attains thickness of 30 m; it is the Engine Vein Conglomerate. These conglomerates are made up of angular grained sands which were laid down in cyclic sequences. This sequence is an upward fining sequence of three or four cycles.

Three other main beds are seen at Alderley Edge. The first is friable mottled sandstone similar to the Upper Mottled Sandstone, the Beacon Lodge Sandstone which attains a thickness of 12 m and can be seen at Beacon Lodge resting above the conglomerates. The second is a conglomerate which overlies the Brynlow and is seen by the site of the old mines and is thus called the West Mine Conglomerate. There are ten upward fining cycles [ cite book| last = Carlon| first = Chris| coauthors =| title = Alderley Edge Mines| publisher = Sherratt, Manchester| date = 1979|location =| pages = 144 pages | id = ISBN 085427053X] in this rock and it has a thickness of 40 m. The third rock is creamy white sandstone, which lacks the pebbly beds and is upward fining; it overlies the West Mine Conglomerate and is seen in outcrops at Brynlow. This is the Wood Mine Sandstone it has a thickness of 16 m. Two other minor beds are seen to the southeast of these areas a single cycle conglomerate/sandstone/marl of 9 m thickness known as the Brynlow conglomerate and a final bed that overlies the Brynlow Conglomerate and is named the Nether Alderley Sandstone. On top of the Nether Alderley Sandstone lie the Mercia Mudstones which attain a thickness of 300 m, but these are only seen on the lower plains, it is in these beds that the halite lies.

Mottling in the red blocky mudstone is of two main types,

* A light red or greyish green coloured irregular sandstone, the lithology of which is different from the main unit. The boundary between this and the main unit is not clear-cut and it is suggested that they are caused by the incorporation of the material whilst the whole was semi-liquid. The incorporated grey owes its colour to the same cause as the grey bands (Not yet fully explained but indications are that the origin was included organic material)
* The second includes the "fish eyes" which occur throughout the marls in the red mudstones. These are small spherical regions in which the iron oxide colouration has been discharged about a centre. A minute speck is usually found in the middle. The colour has been attributed to the decay of small specks of organic matter but recent work suggests that a radioactive centre may be responsible. Other examples of this secondary change of colour from red to greenish-grey are seen along joint planes where the altered zone may only be a fraction of a millimetre thick and sometimes on the cheeks of a gypsum vein.

The ferruginous colour pigmenting is finely disseminated over the surface of individual grains and accounts for a very small percentage of the rock. It is thought that where the rock is grey or grey green either from primary or secondary causes, the oxide has been chemically removed during deposition and thus unmasking the true colour of the minerals in the sediment. The active principle is still unknown.

Conglomerate

The majority of the lower Triassic clastic sediments originated from a Variscan source area in northern France with minor local input. Some Alderley Edge conglomerates, consist of material originating from the Pennine block to the east. Breccias with clasts of local origin, are well developed on the edges of the basin and are interpreted as gravel fans at the mouths of wadis emerging from mountainous areas bordering the depositional basin. Others show long distance river transport by a powerful river system originating in the Amorican Massif. The basal conglomerates are made up of hard sandstone which contains angular grains. At Alderley Edge the junction is quite sharp it could be regarded as an unconformity. However interdigitation is found as if to testify against an unconformity. Mappable beds showing all the characteristics of the Upper Mottled Sandstone are found between the two basal beds of the conglomerates.

The conglomerate is typically a medium to fine grained brown to buff sandstone consisting of angular to sub angular grains with scattered flakes of mica; it makes a good building stone. Conglomerates occur in the basal part of the sequence and shale bands that are micaceous occur in the highest beds. Viewed regionally the conglomerates appear to be impersistent. Near the southern end of the district where the ground is not covered by drift three conglomerates can be mapped. The bottom and most important is mostly pebbles - quartz and quartzites with occasional grits and rarely some igneous. These are set in a medium to coarse grained sand matrix. the pebbles can be up to 100 mm long.

The upper parts of the district is characterised by massive posts of medium to fine sandstone up to 6 m thick separated by chocolate coloured shales which are micaceous. The pots are well bedded and often quarried for building only occasionally are they false bedded.

The basal conglomerate is cut into sections by the faults and can be traced in a general eastwards direction from the village to the hill top 3 km away. It is about 20 m thick and broken into crags along the upper part of the edge. It dips west - southwest at an angle of 8° - 14°.

Marl

Generally the same sedimentary types are present in the marls as are in the waterstones but with less arenaceous and more argillaceous material. An irregular rhythm is discernible, each sedimentary cycle is well developed and have banded or stippled beds at the base. It is in this banded stratum that features such as sun cracks are found. Ripple marks are less common but they are evidence of deposition in shallow water. Pitting sometimes found has been linked to rain pitting but may have occurred during the de-gassing of the marls during drying out. The banded strata which may be red or grey pass up into blocky unstratified mudstones (broken up and allowed to resettle before consolidation).

Tectonics

The Permo-Triassic rocks of the Cheshire basin are heavily faulted. The longer axis of the basin trends towards the NNE, being flanked on the west by the Carboniferous rocks of north Wales and to the east by the Pennine Foothills. The dips in the Permo-Triassic rocks reflect the steady swing of the beds round the north-east edge rim of the Cheshire basin, except to the north of Alderley Edge, where a gentle anticlinal fold centred on Wilmslow plunges westward and is intersected by a number of north-south tension faults.

Crustal extension controls the tectonic accommodation space available for sediments in rift settings [cite journal| last = Ruffell| first = A| coauthors = Shelton R.| title = The control of sedimentary facies by climate during phases of crustal extension: examples from the Triassic of onshore and offshore England and Northern Ireland.| journal = Journal of the Geological Society, London
issue = 156| pages = pages 779–789.| date = 1999| id =| accessdate =] and may be defined by the structural and depositional geometry of sedimentary successions observed on seismic data and the rate of subsidence through time as represented by the accommodation of sediment. The characteristic features of each are dependent on three variables: the time taken for deposition; the interplay between tectonics and eustasy and the lithology (thus facies) of the succession observed. The Sherwood Sandstone Group has been considered to represent a syn-rift phase of fluvial deposition throughout Europe, with the overlying Mercia Mudstone Group interpreted as the succeeding phase of deposition in an evaporitic seaway during post-rift thermal subsidence. More recently, however, [cite journal
last = Tucker
first = Roger M.
coauthors =Tucker, Maurice E.
title = Evidence of synsedimentary tectonic movements in the Triassic halite of Cheshire
journal = Nature
volume = 09 April
issue = 290
date = 1981
pages = pp 495–496accessdate = ] there has been the recognition that it is the Mercia Mudstone Group which is seen to thicken markedly into faults imaged on seismic data rather than the Sherwood Sandstone Group. This work demonstrates the Mercia Mudstone Group to be a syn-rift phase of deposition, with the fine grained nature of the sedimentary record at this time controlled by the prevailing arid climate. Such conditions were not conducive to the large-scale and rapid movement of sediments from the hinterlands raised by relative footwall uplift, thus the sediments are fine grained. The minor thickening of the Sherwood Sandstone Group into faults is interpreted to be a combination of minor extension in the early Triassic superimposed on thermal subsidence inherited from the important regional phase of extension in the early Permian. Analysis of the timing of fault growth indicates a larger proportion of fault-controlled, synsedimentary movement occurred during the mid-to-late Triassic (Mercia Mudstones) rather than the early Triassic (Sherwood).

Post Triassic Folding

Using techniques of seismic sequence stratigraphy the English Permo-Triassic megasequence can be divided into three sequences bounded at the top and base by local or regional unconformities. Construction of a seismic sequence stratigraphy allows the examination of the interaction of tectonics and sedimentation. On this basis the Cheshire Basin can be divided roughly into two areas - a westerly area close to the western basin margin and an easterly area nearer to the Carboniferous WEM / Red Rock Fault. The Red Rock fault - really a group of faults, is the name given locally to one which happens to throw Carboniferous and Permo-Triassic rocks together. It runs north - south a trend followed by the Kirkleyditch and the Alderley faults.. Seismic interpretation shows that deposition in the western area was essentially post-rift - resulting in packages of parallel reflectors on seismic data.

The movements (WEM – Red Rock Fault) which in late Carboniferous times initiated the Rossendale Anticline and the Pennine uplift were repeated later probably during the Alpine of Tertiary age, but the major fold of this date resulted in the Cheshire Basin as it is today, in which the Permo-Trias is preserved. The major faults at Alderley are almost certainly of Tertiary age, their formation following closely the folding episode. These main faults are normal.

The two large faults in the area are the Alderley and Kirkleyditch, which bound the area of interest. [cite journal
last = Warrington
first = G.
title = The Metalliferous Mining District of Alderley Edge, Cheshire
journal = Mercian Geologist
volume = 1
pages = pp 111–131
date = 1965]

*Kirkley Ditch fault - The Keuper sandstone conglomerates dipping WSW at, and to the south of Kirkley Ditch, are clearly detached by a fault from the country to the west underlain by Upper Mottled Sandstone and from the main mass of Keuper conglomerate capping Alderley Edge. A break in gravity gradient near Kirkley Ditch [87307848] supports the inferred position of this fault. To the north and south its line must be conjectural and it is shown as having a roughly north-south trend.*
*Alderley Fault - The original map by Hull and Green showed a normal junction between the lower Keuper Sandstones of Alderley and the marl to the south west. A borehole driven in 1894 at Alderley Edge [NGR SK 84237819] proved Waterstones indicated the intervention of a fault between this position and the Keuper Sandstone to the east. North and south its line must be conjectural. The Edge Fault is visible on the north-east part of the escarpment where it separates the conglomerate on the north from the soft red mottled Upper Mottled Sandstone on the south.

Resources

The red freestone of the Keuper building stone is familiar in buildings and bridges throughout North and West Cheshire. It is easily quarried and yields large free standing blocks and though soft at the time of quarrying, it has quality of hardening on exposure to the weather. The building stone represents a particular lithology of medium grained massive sandstones within the Keuper Sandstone and may not always lie at precisely the same horizon. It seems likely that the rock quarried in the northern part of the district is the lateral equivalent in part at any rate to the conglomerates of Alderley Edge which die out northwards. There are no working quarries of any importance today, but there are many old quarries around the area.

and And Gravel

The widespread deposits of middle sands have been extensively worked around the Cheshire area for building purposes, they vary from clean sharp sand to somewhat loamy deposits with layers of clay. Whilst never quarried in any great quantity at Alderley, the Upper Mottled Sandstone in the main is incoherent enough to be quarried for sand and there are large reserves available at the foot of the Edge escarpment near the Hough,

The Red Sand (moulding sand) seen at Alderley Edge was used extensively for the purpose of constructing the moulds for the foundries in nearby Macclesfield during the late 19C and early 20C.

Copper Ore

Russell who did much of the early work on the Edge geology, mainly in relation to the mining activities said, “It is impossible to give even a rough estimate [of how much ore is available, it can] be said with certainty that the walls of the chambers and drives would yield some tens of thousands of tons of ore averaging 1.3% of copper. ... A more definite statement cannot be made since the ore is exposed on the sides of the workings only and has not been "blocked out", the system in the past having been to follow the richest ... impregnation while leaving the poorer rock standing.” This statement is as true today as when written the distributions of the ore bodies and the fine dissemination makes accurate quantitative analysis difficult. The dissemination would on its own make the working of the ore economically unviable anyway. [cite book|last = [for] Dept of Transport and Industry| first = |title =Minerals in Britain Past Production... Future Potential Copper|publisher = British Geological Survey| date = 1998] Further to the economics are the facts that most of the area is owned by the National Trust and it is designated by Government as a SSSI (Site of Special and Scientific Interest) for its geological value [ [http://www.jncc.gov.uk/page-3582 Joint Nature Conservation Committee] ]

Barytes

Barytes has varied usage and is a valuable mineral. Much of the sandstone at Alderley is baritic; there is presently no commercial potential as it would be too costly to extract the barytes from the sandstone. [cite book|last = [for] Dept of Transport and Industry|first = |title = Minerals in Britain Past Production... Future Potential Barytes| publisher = British Geological Survey| date = 2000|]

Fossils

The environment at the time was not conducive to life so few fossils are found. Occasional minute crustaceans of the genus Euestheria have been found in the marl lenses within the sandstone. [cite journal
last = Warrington
first = G
coauthors =
title = The Occurrence of the Brachiopod Crustacean Euestheria in the Keuper Sandstone of Alderley Edge, Cheshire
journal = Liverpool. Manchester. Geol. Jour
issue =3
pages = pages 315–319
date = 1963
id =
accessdate =]

A few kilometres to the west and further over towards the Triassic outcrops of the Wirral, where conditions were drier, footprints and track ways of insects and small vertebrates, including Rhynchosauroides and Chirotherium, have been identified.

References

General References

*cite book
last = Aikin
first = J
coauthors =
title = A Description of the Country from Thirty to Forty Miles around Manchester
publisher = Reprinted by David and Charles, Newton Abbott, 1968.
date = 1795
location =
pages =
id =
*cite journal
last = Greenwood
first = H W.
coauthors =
title = The Trias of the Macclesfield District
journal = Proceedings Liverpool Geological Society
issue = 12
pages = pp 325–313
date = 1919
id =
accessdate =
*cite journal
last = Macchi
first = L.
coauthors =
title = A Field Guide to the Continental Permo-Triassic Rocks of Cumbria and Northwest Cheshire
journal = Liverpool Geological Society, Liverpool
issue =
pages = 88 pages
date = 1990
id =
accessdate =
*cite journal
last = Taylor
first = B
coauthors = Price R, and Trotter F
title = The geology of the Country around Stockport and Knutsford
journal = Memoir of the Geological Survey GB
issue =
pages = pp 20–22
date = 1963| id =
accessdate =
*cite journal
last = Thompson
first = D. B.
coauthors =
title = The Area Around Manchester - Alderley Edge
journal = [in] Geologists Association Guide No 7
issue =
pages = pp 39 – 51
date = 1970
id =
accessdate =
*cite journal
last = Warrington
first = G.
coauthors =
title = Alderley Edge Mining District
journal = Amateur Geologist
issue =
pages = pp.4–13
date = 1980
id =
accessdate =

External links

* [http://www.derbyscc.org.uk/alderley/geology.htm Derbyshire Caving Club]
* [http://www.alderleyedge.org/Mines/geology.htm Alderley Edge Geology]
* [http://www.cheshire.gov.uk/Planning/NaturalHistoricEnv/Natural/NHE_Natural_Geodiverisity.htm Cheshire County Council]
* [http://www.esci.keele.ac.uk/cheshire-atlas/maps/geology.htm Keele University]