Lepidodendron

There has been a fossilised tree root in Sheffield Botanical Gardens for as long as I can remember. It was not until they put up new signs earlier this year that it's significance became clear.

Clubmosses - fossilised plants that became coal

This is the fossil root of a giant clubmoss called a Lepidodendron that lived about 300 million years ago. It was found in the rocks between the coal seams that were dug out when Sheffield's railway station was being built in the late 1860s. Clubmosses were some of the earliest vascular plants. Today clubmosses still exist but they are small plants that usually grow in boggy areas. Fossils are usually formed when an organism dies and the contents of its cells are replaced with rock forming minerals. This fossil root was surrounded by hardened sediments before the tissues rotted away leaving a hollow mould which was filled with fine sand forming a perfect replica of the original root. During the Carboniferous period Sheffield was covered with vast forests of clubmosses that grew up to 30 meters tall. When they died they piled up and were turned to coal as they were buried and compressed. This coal was mined in the Sheffield area for fuel.

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Ecton copper mine

[Re-post from 23/08/2012 gneisslife.blogspot.com]
On Sunday the walk took us past a disused mine as Ecton. A friend asked if it was a lead mine. I looked it up on google and found out that in fact it was a disused copper mine, the only one in the Peak District.
Ecton copper mine - Ecton mine was the source of the 5th Duke of Devonshire’s fortune and was the deepest mine in the world at that time.
The period of time between 1760 and 1790 was the heyday of this famous Peak District mine. The 5th Duke of Devonshire was the lucky owner when a vast deposit of high grade copper ore was discovered. The ore yielded as much as 60% copper, very high grade indeed, many copper ores that are mined today yield only 2 - 3%. This lasted for 30 years and was the source of money used to build Buxton’s Crescent and Stable Block (the latter was later to become the Devonshire Royal Hospital). Below is an image of a sample of chalcopyrite, photographed near the Ecton Copper mine.
In some places this rich ore body was 50m wide. Deeper in the mine, the copper was increasingly replaced by zinc. The shape of the ore body was described by a Dr. Joseph Watson as being a saddle deposit, this being the name that miners gave to locations where ore bodies had been folded by anticline/syncline formations. Samples of the ores from Ecton mine are held both at Chatsworth house in the Peak District and at the British museum in London. The grotto at Chatsworth is lined with chalcopyrites and calcite from the mine.
Ecton is unusual in the Peak District as it is the only source of copper mineralisation. Some lead and zinc ore has been extracted from this mine also. The mine is the deepest in the Peak District and may have been the deepest in the world at the time (at least 1800 feet deep in 1788). A reason for this record is the nature of the deposits - vertical ‘pipes’. Contemporary writers visiting the mine were obviously impressed by the beauty of the ores - yellow, gold, topaz, violet and azure all being mentioned. The ores responsible for this multifarious appearance were (mainly) chalcopyrites with oxides and carbonates. The gangue was mainly yellow calcite with fluorite and iron minerals.
The mine was worked in the 17th century, possibly earlier but never properly exploited. In 1723 a group of speculators re-opened Ecton mine but unfortunately ran out of money before the rich ore body was discovered. In 1763 and 1764 the output was over 1000 tons per year, although this was reduced by the end of the decade.

The beast of Black Rock

This prehistoric looking beast is an eroded outcrop of Ashover grit at the top of Black Rocks above Cromford (SK293558). The Black Rocks site was mined for lead, the spoil on the scree slope leading up to Black Rocks supports many interesting lead tolerant plants (leadworts). The scree slope is an ideal place to search for a variety of rocks and minerals. Other interesting habitats can be explored from Black Rocks, since it is near the boundary between limestone and gritstone. Cromford Moor has heather moorland and various types of woodland.

From Black Rocks we can see the huge outline of Dene Quarry. It produces crushed limestone for use on roads and in concrete.

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Parkhouse Hill

The following is a re-post of a blog from gneisslife.blogspot.com from 27th June this year. It shows the reef topography of Parkhouse Hill and the geological setting.

A fantastic example of an apron reef at the edge of the Derbyshire Dome limestone plateaux.

Rocks and Scenery of the Peak District

Trevor D. Ford (Landmark Collector's Library)

The first geology of the Peak District book I am going to read is this excellent little book published in 2006. The name of Trevor Ford will come up again and again in this blog as he has published widely on this subject. This book is aimed at the non specialist and is full of fantastic pictures to inspire an interest in geology. I will go through each chapter with a short synopsis a pattern I will repeat for all the books. In this way we can slowly build up our knowledge in bite sized pieces.

The limestones of the white peak

Limestones form the centre of the peak. They formed in warm tropical waters when Britain was near the equator in the Carboniferous. The area was an atoll like structure surrounded by reefs, although these did not form a continuous ring in time or space. The reefs were not formed of coral but algae and indeed some are better described as mud mounds than reefs. There was a lagoonal plateau in the middle.

Toadstones or volcanic lavas and ash falls

Bubbling under this atoll basalt lava seeped out onto the floor. There is also evidence of ash fall, hexagonal jointing and sills. Wow. Toadstone is a local name for this rock type but the origin is unclear.

The Millstone Grit

Surrounding the limestone to the west, north and east are the shale and sandstone sequences of the dark peak. These were formed by deltas resulting from the erosion of large mountains in the north. About 300 million years ago the lagoon subsided and the mountains raised resulting in the cessation of limestone and the deposition of sandstone which eventually covered the limestone.

Structures, folds and faults

There is a large scale anticline covering the whole of the South Pennines with the Derbyshire massif in the middle.

Minerals and mines

There is a long history of mining in the Peak District and we will return to it again and again. Galena, fluorspar, barite and calcite were all mined in the White Peak from deposits laid down at the end of the Carboniferous. Rakes and scrins are vertical fissure deposits with flats and pipes along the bedding or in caves. There is also Blue John but we will leave that for another day.

Before the ice age: the pocket deposits

Around Brassington are pockets of sand formed by the erosion of Triassic sandstones into a limestone basin 7 million years ago. Originally a sheet the pockets formed as the limestone eroded. They form an unusual landscape as the heathland vegetation of the silica pockets contrasts with the limestone vegetation.

The great ice age

The last ice advance, the Devensian (80-10 kya), did not reach the Peak District stopping only as far south as York and South Staffordshire. The earlier Anglian (470-420 kya) did reach the Peak but the flow was too slow to carve the U-shaped vallys.

The remaining 10 chapters are all short reports about more geomorphological features such as rivers, caves, dry valleys, landslips etc all of which I am sure we will have lots more to say about in the coming months.

This book is now out of print but it features many colour photos and geological sketch maps and is well worth getting hold of.

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Alport Castles

One of the finest views of regular medium bedding is at Alport Castles which forms a fantastic natural amphitheatre.

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River Ashop syncline

This syncline is found on the River Ashop in the grid cell below (SK143894) alongside the A57.