Permission for this Earthcache has been granted by Hanson UK. Permission has also been granted by Natural England because it is a Site of Special Scientific Interest (SSSI).
The grey rocks at the bottom of the quarry face are Carboniferous limestones deposited about 340 million years ago. These rocks are made up of fine grained calcium carbonate and the skeletons of marine animals, like brachiopods, corals and algae. The area was close to the Equator at this time. After the animals died, their shells became buried in the fine grained calcite “mud” (i.e. calcite, not clay mud) to form the limestone.
Around 290 million years ago, these Carboniferous rocks were uplifted, folded and faulted by plate tectonic movement. You can see that these grey rocks are tilted to the left as you look at the rock face.
During a long period of geological time the folded Carboniferous rocks in this area were subjected to weathering, erosion, and then further deposition and re-erosion. The whole story is told in the series of Earthcaches Mendip Hills 1 - 4.
As the sea advanced and retreated around the Mendip islands during Early and Middle Jurassic times, the sea floor was probably buried many times by sediment and then re-exposed by erosion. Whenever conditions were appropriate, the sea floor would have been colonised by rock boring and encrusting animals. Such surfaces are called ‘rockgrounds’ and can be seen at the Earthcache Tedbury Camp - Jurassic Sea Bed. By around 175 million years ago the area had been eroded to an almost level rock platform and plate tectonic movement meant that the plate was now in present-day Mediterranean latitudes, about 30 degrees north.
Jurassic limestones were then deposited on this sea bed and these are the yellow, creamy coloured rocks you can see above the horizontal surface at the top of the grey rocks. They are called oolitic limestones because they contain tiny spherical grains called ooids or ooliths. These form in highly agitated, strongly evaporating sea water of depths up to about 20m. They are forming today off the coast of the Bahamas. The limestones here also contain shelly fragments of marine bivalves, belemnites and ammonites, many species of which had not evolved when the grey limestones were deposited.
Much later these rocks were uplifted but there is very little evidence of folding or faulting at this site. Further south the effects of this plate tectonic folding were more severe, creating the Alpine ranges of fold mountains across southern Europe.
No doubt there were many thousands of metres of later rocks deposited here but there is no evidence of rocks younger than the yellowish Jurassic limestones at this site. Those that were deposited have been lost by ancient periods of weathering and erosion. This site today is about 100 metres above sea level and is undergoing modern day processes of weathering, soil formation and erosion. Modern rivers, like the Mells River, have cut through the overlying Jurassic limestones in places to reveal the folded Carboniferous beds beneath.
Quarrying (biological weathering?) has recently exposed the quarry faces. In time, if human action does not intervene, the weathered material from this area will find its way into the Mells River and be transported to the River Avon, and eventually become part of the modern day sediments in the Severn Estuary. The rock cycle continues!
What to do
1. Estimate the tilt or dip of this rock, i.e. the angle from the horizontal.
2. In which direction are the rocks tilted?
3. As this is an SSSI, no hammering of the rocks is allowed so please find a loose piece of the grey Carboniferous limestone on the ground. Describe its grain size compared with grains of sand.
4. Search for fossils. You may be lucky and find brachiopods or corals.
5. Search for a piece of the yellow Jurassic limestone. Do not climb the rock face to find one. The Jurassic limestone can be more easily investigated at the nearby Earthcache Tedbury Camp - Jurassic sea bed . If successful, describe its grain size compared with grains of sand.
Please email your answers to me, JurassicEdie
Return route:
N51º 14.443 W002º 21.103 - walk to bridge (do not cross) and take path to right
N51º 14.470 W002º 21.180 – overgrown lime kiln on right – walk past; there is a better one further on
N51º 14.492 W002º 21.218 - This is the site of a renovated lime kiln. Layers of limestone and wood or coal were put into the kiln and the fuel was burned until a temperature of 900 - 1000º C was reached. The quicklime would be raked out at the bottom; a very dangerous job. This was spread on to farmland to reduce the soil's acidity.
Proceed to N51º 14.564 W002º 21.193 where you will join the path back to the bridge at Great Elm.