For readers who want an overview, the author suggests they read the introduction and summary sections. However, this page goes into more detail which will suit those with a greater interest in geology.
The Stratigraphy of Alderley Edge
Put simply, the interior of the Earth is separated into three zones: a core, mantle and crust (fig. 1). The core is composed of a solid inner zone which is surrounded by a liquid outer zone. At a depth of ca. 2,900 km the outer core meets the mantle, a zone within the Earth which behaves in a semi-plastic manner. The outermost layer of the Earth is divided into continental crust (about 35 km thick) and oceanic crust (about 10 km thick).
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Figure 1: The Earth's interior |
The crust is divided into a series of tectonic plates which 'float' on top of the asthenosphere, a part of the mantle where temperature and pressure reduces the strength of the rocks, causing them to 'flow' or move plastically. Tectonic plates move across the surface of the Earth due to convection currents in the asthenosphere. They meet at plate boundaries which can be either convergent, divergent or transform. A convergent plate margin is when oceanic crust meets continental crust or oceanic crust meets oceanic crust. In the former case, due to its greater density, the oceanic crust is subducted (sinks) beneath the continental crust where it melts and becomes magma. To maintain balance, oceanic crust is formed at divergent plate boundaries where magma rises through fractures and solidifies. At a transform margin the plates move past one another along transform faults (the San Andreas Fault is a prime example).
The result of plate tectonic activity is that the continental crust moves across the surface of the Earth over millions of years in much the same manner as a beach ball on the ocean. During the Triassic the British Isles (which are part of the Eurasion plate) were situated about 20° N of the equator. During this period the climate was harsh and a desert covered the Cheshire Basin. Life in this environment was only possible because of the wet season which replenished dried riverbeds during periods of violent storms.
The Stratigraphy of Alderley Edge
There are three rock types that together form the Earth's crust: sedimentary, ign
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Figure 2: The geological column |
eous and metamorphic. Sedimentary rocks are formed by the
lithifaction of accumulated sediments on the Earth's
surface, and normally form layers (sedimentary beds) which can
vary greatly in thickness. Igneous rocks are formed when magma solidifies either
below the Earth's surface (intrusive) or on the Earth's surface (extrusive).
Metamorphic rocks are the product of any existing rock type which has been
altered by heat and pressure without melting taking place.
Alderley Edge is comprised of sedimentary rocks which are approximately 240 million years old which makes them Lower Triassic in age (fig. 2). To put this into context, table 1 lists some of the major events in the Earth's history.
Table 1: Major events in the Earth's history
| Event | Time (approx) |
| First hominids | 3.5 million years ago (Neogene) |
| Dinosaurs became extinct | 65 million years ago (end of Cretaceous) |
| First dinosaurs appeared | 230 million years ago (Triassic) |
| First terrestrial life | 415 million years ago (Silurian) |
| First multicellular organisms | 1 billion years ago (Precambrian) |
| Earth formed | 4.6 billion years ago (Precambrian) |
The rocks which comprise Alderley Edge are a series of sandstones, conglomerates and siltstones which represent a variety of depositional conditions. These rocks are arranged into different formations due to variations in their physical properties (fig. 3). Organising rocks in this way is known as stratigraphy. Figure 4 is a geological map of the area showing the distribution of the various rock types and the structure of the Edge. Each stratigraphic formation will be described below in terms of its appearance and depositional setting.
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| Figure 3: The
stratigraphy of Alderley Edge |
Figure 4: Geological map of Alderley Edge |
Wilmslow Sandstone (formerly Bunter Upper Mottled Sandstone)
The Wilmslow Sandstone Formation contains the oldest rocks in the area and consists of aeolian (wind deposited) and fluvial (water deposited) sandstones. In both cases the rock is fine grained and is a rusty red colour because the iron content was oxidised during deposition (fig. 5). There is localised white colouration in the sandstone due to the presence of barite, a mineral which removed the rock's iron content during formation [3]. The aeolian sandstones are predominantly composed of quartz grains which display a rounded shape. The quartz grains continually collided with each other during transportation which resulted in their rounded appearance. Trough cross bedding can be seen in the aeolian sandstones which represent the internal structure of ancient dune formations (fig. 6).
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| Figure 5: Wilmslow Sandstone displaying red colouration | Figure 6: Trough cross bedding, Mottram Quarry |
In contrast, the fluvial sandstones consist of more angular grains and also contain the mineral muscovite. The fine grained nature of the rock and presence of muscovite suggest that it was deposited in a calm body of water such as the bend of a meandering river or an isolated pool. Following deposition the sediment was exposed to the air which caused the iron content to oxidise and turn red. Therefore it is likely that the body of water which deposited the sediment evaporated in a relatively short period of time.
The Wilmslow Sandstone is exposed at Mottram Quarry and Pillar Mine amongst other places.
Engine Vein Conglomerate
The Engine Vein Conglomerate Formation (EVC) contains layers of conglomerate, sandstone and mudstone but these three rock types are not always exposed at the same locality (fig. 7). When a layer of conglomerate is overlain by a layer of sandstone, and the
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Figure 7: EVC underlain by Wilmslow Sandstone, Castle Rock |
sandstone layer is
overlain by a layer of mudstone, these three layers are collectively known as a
fining-upwards cycle. A fining-upwards cycle is so named because the grain size
decreases as you move upwards through the sequence and represents the gradual
drying up of a river. When the wet season begins, and the river is in flood, the
high energy involved can transport anything up to the size of large pebbles (in
this instance). As rainfall diminishes, and the river begins to lose some of
it's energy, the heaviest fragments of rock are deposited and form the
conglomerate. In calmer areas of the river (such as the inside of a bend) the
finer sediment is deposited, which is represented by the sandstone layers. Where
the river burst its banks the finest sediment is deposited due to the absence
of currents and eventually forms the mudstone layers. The rivers which cut
across the Alderley Edge area during the Triassic flowed northwestwards [3].
The layers of conglomerate are a pale grey colour and contain various inclusions (pebbles etc.) in a fine grained matrix (fig. 8). The inclusions are quartzite and various igneous rocks which range in length from 0.5 - 10 cm and are rounded in shape. Flakes of muscovite are present in the matrix and give the rock a sugary appearance. The rounded nature of the inclusions and the presence of muscovite indicate a fluvial origin.
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| Figure 8: Close up of EVC |
The sandstone is a fine grained pale yellow/grey rock predominantly composed of angular quartz grains. However, flakes of biotite and muscovite are present and can be seen through a hand lens. Trough cross bedding has been recorded in these rocks by Thompson which represent the internal structure of ripples on the riverbed [4]. The presence of biotite, muscovite and cross bedding prove that these sandstones are fluvial.
Thompson notes that the mudstone is red and green at Engine Vein Quarry with mudcracks and ripple marks preserved [4]. The presence of mudcracks indicate that the mud and silt was exposed to the sun for some time after the pool of water had evaporated.
The best places to see EVC exposed is at Castle Rock and Stormy Point.
Beacon Lodge Sandstone
There are no notable exposures of the Beacon Lodge Sandstone (BLS) but Carlon notes that it is a red mottled sandstone which is similar to the Wilmslow Sandstone [2]. As its name implies this rock formation underlies the area around Beacon Lodge and is around 12 metres thick [2]. It is likely that the BLS represents a similar environment to the Wilmslow Sandstone. Therefore the climate had become more arid after the wet period which deposited the EVC.
Wood Mine Conglomerate
The Wood Mine Conglomerate (WMC) is similar to the EVC but is situated above the Beacon Lodge Sandstone (fig. 3).
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Figure 9: Exposure of WMC, Church Quarry |
Layers of
conglomerate, sandstone and mudstone comprise the WMC and form fining-upwards
cycles in the same way as noted in the EVC. Therefore these two rock formations
were deposited in very similar conditions. The Wood Mine is excavated into WMC
and ten fining-upwards cycles have been recorded in the workings [2].
The Church Quarry displays good exposures of WMC (fig. 9).
West Mine Sandstone
The West Mine Sandstone Formation (WMS) contains both aeolian and fluvial sandstones but lacks layers of conglomerate and mudstone. The sandstone is fine grained and displays a pale yellow colouration due to the presence of barite. These rocks are composed predominantly of quartz grains which are sub-angular to rounded in appearance. In places the sandstone displays a millet seed texture which indicates an aeolian origin [3].
A small exposure of WMS can be seen in Brynlow Valley (fig. 10).
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Figure 10: An exposure of weathered WMS in Brynlow Valley |
Brynlow Conglomerate
One fining-upwards cycle of conglomerate, sandstone and mudstone comprises the Brynlow Conglomerate Formation. It rests above the WMS and represents a relatively short period when the climate reverted to semi-arid conditions (a wet and dry season).
Nether Alderley Sandstone
Outcropping at Brynlow Quarry (fig. 11), the Nether Alderley Sandstone Formation is composed of a fine grained pale yellow sandstone. It represents both aeolian and fluvial conditions and is similar in appearance to the WMS.
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Figure 11: Nether Alderley Sandstone exposed at Brynlow Quarry |
Tarporley Siltstone & Lower Mudstone
These formations represent a tidal flat environment and contain fine grained sandstones and mudstones. It is these rocks which form the majority of the Cheshire Plain and contain the famous Cheshire rock salt [2]. The rock salt formed when isolated bodies of seawater evaporated as the inland sea retreated.
The Tarporley Siltstone is exposed at Butts Quarry as fine grained purple sandstones and mudstones (fig. 12).
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| Figure 12: Tarporley Siltstone exposed at Butts Quarry |
The Triassic rocks of Britain are notoriously devoid of fossils because they represent a harsh environment which was not conducive to life or fossil preservation. However, the fossils which have been found at Alderley Edge and the surrounding area are discussed here.
Specimens of the brachiopod Euestheria minuta (fig. 13) have been found at the Old Alderley Quarry [5] and Engine Vein Quarry
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Figure 13: A sketch of the brachiopod Euestheria minuta [2] |
[6]. In both
cases the fossils were discovered in a layer of red mudstone which suggests the
brachiopods lived in a calm pool of water. To begin with the water would have
been fairly fresh but as the water evaporated it became brackish which possibly
stunted the brachiopods growth. During the final stages of evaporation the
remaining water would have had a high salinity content, which would have been
the likely cause of the brachiopods death [7]. Euestheria minuta
have also been discovered at nearby Styal along with an insect wing [7].
Rhynchosaur footprints have been found in the surrounding area which were produced by small herbivorous reptiles (fig. 14). A Chirotherium footprint has also been discovered from a borehole at Wilmslow [8] which was produced by a large carnivorous reptile (up to 2.5 metres long). Figure 14 displays a reconstruction of the Chirotherium footprint producer. A three-toed reptile footprint was discovered near the Wizard's Well by Geoffrey Warrington who also discovered plant microfossils at Engine Vein Quarry [2].
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Figure 14: A reconstruction of the Chirotherium producer and a rhynchosaur (inset) |
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The rocks of Alderley Edge and the surrounding area were folded into a gentle anticline (arch) after deposition due to movements in the Earth's crust. The fold is known as the Wilmslow Anticline and the rocks of Alderley Edge are situated on its southern limb [2]. As a result the sedimentary layers which comprise Alderley Edge dip towards the southwest at an angle between ca. 10 - 15°.
After the episode of folding, the rocks of Alderley Edge were uplifted by faulting which was again caused by movements within the Earth's crust.
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Figure 15: Veins of barite in Wilmslow Sandstone |
There is some contradiction in the literature as to when the faulting
took place. Warrington mentions that the area was
faulted during the Jurassic (205 - 146 Ma)[9] whereas Carlon believes
it took place during the late Tertiary (11.5 - 2 Ma)[2]. The Alderley
and Kirkleyditch Faults bound the Edge to the west and east respectively. The
rock in between these two faults was uplifted as a block (known as a horst) and
endured another episode of faulting. As a result the horst is traversed by
several WNW - ESE trending faults which have caused a series of less significant
vertical movements [2]. A steeply inclined smooth surface (called a
fault plane) is associated with faulting and represents where the movement
occurred. A good example of a fault plane can be seen in Wood Mine. The rocks in
the immediate vicinity of the fault plane are deformed into a clay-like rock
called fault breccia. At Alderley
Edge the location of a fault is marked by veins of the mineral barite which can
be seen at Stormy Point amongst other places (fig. 15).
During the last Ice Age, the Edge was surrounded by ice and a lake formed below Stormy Point which flattened the ground. When the climate warmed up the ice melted and a steep sided valley was created in Waterfall Wood by the large quantities of melt water flowing through the area [10].
The most abundant ore minerals found at Alderley Edge and Mottram are malachite, azurite, chrysocolla, galena and asbolite (a comprehensive list can be found on the DCC's website). A brief description of each mineral can be found in table 2. The ore minerals are found in the EVC, WMC, WMS and the upper part of the Wilmslow Sandstone Formations. They are concentrated around the WNW - ESE trending faults although secondary (altered) copper bearing minerals are more widely dispersed due to redeposition (see epigenetic origins below) [11].
Table 2: A description of the common ore minerals found at Alderley Edge and Mottram
| Mineral | Description |
| Malachite |
The principal ore of copper, malachite is a green mineral which in this case forms thin layers. It coats and often cements the quartz grains and pebbles [2] |
| Azurite |
Azurite and chrysocolla are both copper ores, the former being blue and the latter blue-green in colouration. They are commonly associated with the WNW - ESE trending faults [2] |
| Chrysocolla | |
| Galena |
Galena is the main lead ore at Alderley and is a silvery-grey colour. It is commonly associated with barite near to the WNW - ESE trending faults [2] |
| Asbolite | Asbolite is the chief cobalt ore and is often found together with malachite. The greatest concentration of asbolite is in the WMC where it is associated with faulting. It is black in appearance [2] |
Origin of the minerals
There are two theories which geologists have put forward to explain the origin of the mineral deposits around Alderley Edge and Mottram. The first is the syngenetic theory which states that the minerals were deposited at the same time as the Triassic host rocks [2]. Geologists believed that the source of the minerals was nearby metal-bearing highlands and that water transported the mineral substances to Alderley [12]. The syngenetic theory was most popular during the 19th century but more recently views have changed and most geologists now accept that the minerals were deposited epigenetically. The epigenetic theory basically states that the minerals were deposited long after the formation of the host rock from fluids entering the fault system [2]. A more detailed description of the epigenetic theory is outlined below.
Epigenetic origins
It is believed that the carboniferous shales of the Cheshire Basin are the source of the copper, lead, zinc, iron and barium found at Alderley and Mottram. The metal ions contained in the shale were dissolved by fluids from the Triassic rocks and transported to the Alderley area [11]. When the fluids reached the impermeable mudstone layers and fault breccia they became trapped which concentrated the mineral deposits in these areas [2]. The overlying impermeable Tarporley Siltstone and Lower Mudstone Formations acted as 'seal' which prevented further migration of the fluids [2].
The copper and lead minerals were formed by the precipitation of fluids rich in methane, whereas the barium was precipitated separately from fluids with no organic content to form the extensive barite mineral veins [11]. Finally, the primary unaltered copper (and to a lesser extent lead) minerals were redeposited through the porous sandstones and conglomerates by weak carbonic acid, which was formed by rainfall reacting with organic matter on the surface [13]. As a result the secondary copper minerals are the most widely distributed mineral deposits at Alderley and Mottram.
Alderley Edge is composed of Lower Triassic sedimentary rocks which represent a semi-arid desert which suffered seasonal flash floods. These rocks were deposited as horizontal layers which now dip gently to the southeast due to subsequent folding. Fossil evidence indicates that ancient reptiles, insects and crustaceans struggled to survive in the harsh environment. Following deposition the layers of rock were uplifed (due to localised faulting) and eroded which has resulted in the formation of the Edge. Fluids rich in trace elements rose up through the faults and deposited the copper and lead minerals amongst others. These minerals are undergoing alteration to this day, forming a suite of more widely distributed secondary minerals.