Introduction to the geology of Brown's Folly Nature Reserve
Brown’s Folly Nature Reserve consists of 91 acres of land on the hillside overlooking the valley of the River Avon about half a mile south-east of the village of Bathford. It is owned by the Avon Wildlife Trust and has been designated as a site of special scientific interest (SSSI) because of the Middle Jurassic rocks, plants and wildlife that can be found there. The EarthCache is here with the permission of The Reserves Manager, Avon Wildlife Trust
It is a very interesting area to visit as extensive limestone mining was carried out in the past, the stone being used for building and known as Farleigh Stone, a variety of the famous 'Bath Stone'. It was used extensively in the buildings of the Georgian City of Bath. You will see the remains of lots of adit mines which were short tunnels dug into the hillside to extract as much stone as was possible and safe. This activity, long discontinued, has left the rock exposed at many sites enabling it to be examined. The study of the different layers (strata) of rock and the fossils they contain has made it possible to establish when they were formed and to build up a picture of what the area would have looked like at the time.
The rock is about 165 million years old and when it formed the area was covered in a shallow (15 - 20m), warm sea. Tiny fragments of shell were rolled back and forth in the currents in the sea and they became coated in concentric layers of calcium carbonate precipitated from the sea water. When the rounded ooids (or ooliths) were too heavy for the currents to support, they fell to the sea bed and, sometimes mixed with shell material from organisms living in the sea, became cemented together to form the oolitic limestone rock we see today. As conditions such as the depth of the sea varied, different types of sediment formed to be later consolidated into the different rocks seen today on the reserve. Rocks likes these are forming today off the Bahamas and the Indonesian islands
The rocks seen on the reserve are from the Chalfield Oolite Formation and the Forest Marble Formation. A number of different rocks make up each group and these can be seen in the diagram below, 'Sequence of Rocks at Brown's Folly'. In this table, the rocks are set out in sequence, together with their total thickness and an indication of the rocks that can be seen at each site. 13 sites make up the geological trail at Brown's Folly.
Safety precautions:
Most of the sites are easily accessible but there are some steep slopes and rough paths.
There is always the possibility, however small, of rocks becoming dislodged from any rock face whether natural or artificial, and visitors should take sensible precautions, such as not standing directly under or attempting to climb high faces, to avoid putting themselves at risk. If samples of the various rocks and fossils are required, they should be sought amongst the loose rocks at the base of the rock face. Please do not hammer at the rock faces.
From the car park proceed to Site 1 the EarthCache - N 51 23.730' W 002 17.728'
The site is formed by the two walls of an access route to an old mine. It was probably originally an adit mine, (a mine into the hillside) the roof of which collapsed and was cleared. The bottom 2m of the rock faces shows the cream stone, now very fractured, that was used for building. This is the Bath Oolite or Farleigh Down Stone. If this is examined carefully (a hand lens helps) the tiny spherical ooids, or the hollows where they have been, can be seen.
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Oolitic limestone showing ooids and
pieces of bivalve shells in a calcite cement |
Have a look for fossils in the rock face and fallen blocks. It is unlikely that you will find anything other than shell fragments as the constantly moving water in which the ooids formed was not conducive to marine life.
Above the Bath Oolite is younger rock, a band of hard, fine grained limestone, the lower part of the Upper Rags. This was called Roof Bed by the quarry men as it formed a strong roof to the mines. There is a small ‘cave’ where the oolite has been removed from below the Roof Bed. The rock forming the Roof Bed is clearly different from the oolite below, and, although it is not easy to examine it in detail, it does contain a variety of fossils including corals and is referred to as the Coral Bed by geologists. For this rock to form, the agitated water and constantly moving ooid sediment was replaced by a more stable environment in which marine organisms could live.
Above the Roof Bed in some places, white oolite (Upper Rags) can be seen indicating that there was a change back to the type of environment in which Bath Oolite formed. This white oolite is the youngest of the three rock types seen here. We know that because it is on the top and we know the rocks have not been overturned.
The Roof Bed acts as a good ‘marker band’ in the rock sequence and can be seen at a number of other sites on the reserve.
On the west wall there is a small patch of rocks consisting of cemented rock fragments and some pebbles. This is very much younger than the rest of the rock having been deposited probably less than a million years ago and results from rock fragments washing down into rock fissures and becoming cemented. Take great care not to damage this rock.
Also in the west wall low down in the Bath Oolite holes made by the quarry men, possibly for fixing Lewis bolts, can be seen. Notice that the layers of rock, where not fractured, are horizontal (flat-lying).
It is fun to imagine what it was like to be there when the oolitic sediment was forming. You would need breathing equipment as you would be swimming in warm, quite shallow sea water and being buffetted around by strong currents. The water would have lots of ooids suspended in it so would appear quite murky.
What to do
1. Choose an unweathered (cream) piece of Bath stone (lots of pieces have fallen to the ground). Describe the rock carefully.
2. Explain what animals would have been living in the sea when the rock was forming c.165 million years ago.
3. What animals would have been in deep water, on land, in the air at this time in the Jurassic? Would you want to go back in a time machine?
4. Bath Oolite is a different rock from the Roof Bed. What change in environment caused that difference?
5. Take a photo of yourself in this old adit mine (optional).
Email your answers to me, JurassicEdie