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Potteries Orienteering Club

West Midlands Orienteering Association

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First published in The Potter March 2013
Text © Copyright John Heaton 2013

POTOC Geology

I have always had a reasonable knowledge of local geology, having trained as mining engineer. But whilst trying to determine the rock type of a mountain in Wales, I stumbled upon the British Geological Survey (B.G.S.) website which has recently created an open access map. This is interactive and zoomable down to 1:25000. Using this I was able to find out more detail on those areas outside the coal measures for which I had little knowledge.

To understand the geology of our areas it helps to know the geological sequence of the strata in this part of the world.

Apart from the red blobs, which will be explained later, all of the bedrock was laid down between 208 and 327 million years ago. The limestones of the White Peak were formed before this time . Due to major fault lines in the strata, rocks of the Permian age have been thrust down, and are not present locally. Probably the nearest Permian rocks – mainly sandstones – are around Kinver at the very southern tip of Staffordshire.

Geological Strata in the Potteries

Triassic

The rocks of this period are mainly sandstones and typically a reddish colour.

Geologists have identified two separate zones of this era, one centered around Cheshire and the other the West Midlands.

These overlap in North Staffordshire. I can tick off a good proportion of our areas that lie on strata called the Kidderminster Beds, obviously part of the Midland zone; Swynnerton, Trentham, Tittensor Chase, Downs Banks, Kibblestone, Maer Hills and incidentally all of Cannock Chase all lie on bedrock of Kidderminster Conglomerate (pebbly sandstone) and Kidderminster Sandstones. As the name implies this bedrock extends from just south of Kidderminster running N-S. Stone and Stone Common lie on bedrock of Triassic rock, which is not, the predominant sandstone called Mercia Mudstones. The western half of Swynnerton lies on Sandstone called Wildmoor Sandstone.

Beyond Swynnerton and Trentham all of the Triassic rocks were formed in the Cheshire basin zone. The remaining areas that lie exclusively on these Triassic sandstones are Park Hall and Weston Heath. Geologists have named the rocks at Park Hall the Hulme Conglomerates. Weston Heath lies on bedrock of Wilmslow Conglomerates. The centre of the town at Leek lies on an island of bedrock called the Chester Conglomerates, which is close in the sequence with the Wilmslow rocks. Both Ladderedge and Brough Park lie partially on Chester rocks. The western half of Ladderedge and the eastern half of Brough Park are on bedrock much older mudstones or siltstones formed during the middle Carboniferous, shown in the diagram above as “Carboniferous Gritstones”.

Carboniferous Coal Measures

Cross-section of the Potteries Coal Measures

Despite the Triassic rocks lying on top of the rocks in the Gritstone series at Leek, the Coal Measures are next in the chronological sequence (ignoring the 40 million year gap of the Permian period).

A very good place to see the full range of the rocks in this series is at Apedale. Opposite the Heritage Centre and Museum there is a 50 m long and about 15 m high crag, which as well as showing the range of strata, shows a good view of a major fault causing a displacement of the seams. I have always thought it was one of the main faults in North Staffs, appropriately called the Apedale Fault, but having a browse around the museum after the recent Local event, I discovered that it is a short off shoot of the main fault, which actually runs behind the Museum. On the LHS of the crag a coal seam and dark grey shale and mudstones can be seen. On the RHS bands are very pale sandstone outcrops. The line of the fault is covered in vegetation, so is not fully visible. On the diagram above, Apedale lies on the far left on the Potteries Anticline. The Far East section of Apedale lies over that part of the non-productive strata called the Etruria Measures. Non productive refers to absence of coal seams.

Also lying in the Coal Measures are Burslem Park, Tunstall Park, Hanley Forest Park, Hanley Park, Bathpool & Birchenwood, Chatterley Whitfield and Biddulph Grange. Areas that lie on the Etruria Measures are Festival Park and Florence Park. The other part of the non-productive are the Halesowen Measures, formerly known locally as Old Red Sandstone, to distinguish it from the similar Triassic red Sandstones. Keele University and Longton Park lie on this bedrock.

Carboniferous Gritstones

Gritstones are a very hard, dark coloured sandstone.

As stated above Biddulph Grange lies on Coal Measures bedrock, The rocks are at the very base of the measures. The west end at the top of the hill and the northern edge alongside the stream lie on strata at the top of the Gritstone beds called the Rough Rock, which can be seen in the many outcrops. The strata at The Roaches lie on three different seams in this series, one called Milestone Grit, Chatsworth Grit and its very own Roaches Grit. Ramshaw Rocks lies only on the Chatsworth Grits. The latter is the most prolific bedrock in this series, The Cloud, Mow Cop, Wetley Rocks, Shutlingsloe, Teggs Nose and all of the Eastern Edges from Chatsworth to Stanage Edge outcrop this rock.

Volcanic Activity at Swynnerton

In 1842, during his time at Maer Hall and when he was not wandering around Maer Hills, Charles Darwin was wandering around the village of Butterton. He was a keen geologist and was investigating unusual rocks in the area. It was then that he identified that these rocks were part of a volcanic dyke. It became known as the Darwin Dyke, but is now known as the Butterton-Swynnerton Dyke. The Dyke is a vertical column cutting through the surrounding strata to an unknown depth. It is about 10ft wide and runs from Butterton in a southerly direction to terminate in the fields opposite Swynnerton Army Camp. En route it passes through the eastern side of Swynnerton Old Park, where it actually splits in two. The rock has been correlated to the volcano that formed the Cuillins on Isle of Skye. This activity took place between 20 and 65 million years ago. As many of you will know The Cuillins is an area of high magnetic disturbance, where compasses are not reliable. Magnetic detectors have been used to accurately trace the line of the dyke. However as I walked along the track from the Car Park past the underground reservoir where I knew the twin dyke passes beneath, the magnetic effect is not strong enough to deflect a compass. I paid two visits to the area, on the first I examined all the crags in the vicinity of the line of the dyke to look for an outcrop. I did not find any, however the track next to the underground reservoir has recently had the surface edges scraped clean for forestry vehicle access, and this has exposed numerous fragments of the dyke. The dyke is only 8-10 ft wide, but the strata either side for at least that distance has been metamorphosed by the heat of the eruption. The dyke is composed of a olive green gabbro, but the sandstones have melted to form multi-coloured opaque glass like rocks and light pock marked rocks which when split emit sulphurous gases trapped inside. I made a brief return a few weeks later, with my compass to test the magnetic effect. On both occasions I saw two groups of people with metal detectors. I wondered if the dyke was giving them false trails. I did quiz one man, but he was very evasive.

Volcanic Activity at Apedale

In 1920/1 an exploratory borehole was drilled somewhere near the north edge of Apedale Country Park seeking to tap into potential oil reserves. Oil is more often found in an anticline of porous rock such as sandstone trapped below non-porous strata. Unfortunately after about 400 m, instead of striking oil they found volcanic rocks, which continued for another 800 m. The rocks have been identified as occurring during the Carboniferous not long after the surrounding strata was formed and a correlation has been made with the Volcanic rocks in Devon.

John Heaton