FOGGY HILL STROLL IN THE EBBOR GORGE REGION February 19th & October 22nd 2018.

This report is a compilation of two visits to Deer Leap in very foggy conditions in February and repeated in October when we were able to appreciate the wonderful views! Most of the photos are from the second fieldtrip whilst the text is a combination of the two field trips.

Introduction.

The walk was on the southern part of a crunched and folded area of the Mendips known the Variscan Foreland fold thrust belt, formed about 300 Ma when the region lay astride the equator. Experts say that were it not for this thrusting (crunching) of the anticlinal folds Bristol would have been at least 20 kilometres further from Wells than it is today.

In our group, on the first visit, we had the advantage of a local land owner whose land we were exploring, and has an interest in Geology. Despite fairly dense hill cloud preventing any views more than 50 yards this was a fascinating stroll around the hills above . Close to and above the car park we found ourselves perched on four or five different areas of geological interest:

1. Black Rock Limestone. 2. Burrington Oolite 3. Clifton Down/Hotwells Limestone. 4.Oxwich Head/Hotwells 5. Quartzitic Sandstone of the Millstone Grit (this sandstone in places in red and I think that probably caused confusion in listing it as an extra unit) 6. Coal Measure Mudstones to be concentrated in the sharp upturned ridge of the Ebor thrust. (in age order, oldest first)

The area of the walk had been the site of a medieval farm settlement with not much evidence of dwellings but various broad pits to show sites of farm buildings and the odd stone implanted manually into the ground nearby.

We started in Deer Leap carpark standing on the southern limb of a pericline fold trending east west and dipping south in front of us and walked down the exposures with the beds dipping south. These are four layers of with the oldest at the top of the slope being Black Rock limestone, then Burrington oolite, Clifton Down and finally Oxwich Head/Hotwells. The Devonian rocks are to the north. Below that, uniquely for southern Mendips, are coal measures. There are plenty of coal measures in the northern Mendips. In this area they have been mainly eroded away. In 1800’s potential coal measures in this area created interest and pit shafts were put down but failed as the beds only a few centimetres thick.

Site#1 Clifton Down Limestone

The bedding planes were very obvious at this outcrop of Clifton Down limestone. We could see slight weathering from acid rain along the joints slightly but it was not as extensive as in areas like . Here weathering is partly restricted as CDL it is dolomitic limestone and the calcium in the calcite has been replaced by magnesium which is not as dissolvable. We know it is Clifton Down limestone because of the fossils found in the rock. The dip of 60 degrees (way the rocks are inclined) is quite steep and the strike 130 (where it extends to e.g. on the map) is at right angles to the dip. It is useful to use water to see the angle of dip.

Stop # 2. Oxwich Head/Hotwells Limestone

The Oxwich Head /Hotwells limestone as this site shows much more weathering. This karst weathering is because this is a pure calcite limestone. At the previous site there was more dolomite in the CDL hence less obvious weathering. The roughness here is due to secondary weathering. The difference in age between CDL (340million) and OH (330 million) is 10million year yet we found the outcrops within a few metres of each other.

Stop # 3.

We scrambled down a few metres to find another outcrop, which looked quite different from the limestone above. It was jointed, with limited solution weathering and much lighter in colour as, unlike some sandstones, it contains no iron. This rock is called Quartzitic Sandstone and was hard with angular outcrop shapes and when broken had conchoidal sharp edges. It marks the boundary between the lower and upper carboniferous and was deposited in shallow waters of a tropical clear sea with sediment brought in from a delta spreading from the north. Continental drift brought this new source of debris but it is not known what! It is a bit of a mystery.

This hard and brittle quartzitic sandstone had been re-crystallised due to it being heated at some stage a long time ago. There being no evidence of igneous rocks locally to suggest volcanic activity so possibly this heat may have come from the friction of the shearing during the thrusting and formation of the anticlinal folds which created the so long ago.

Stop # 4 Only a few metres to the north we came across a darker stone formation called Black Rock Limestone. Occasionally some intervening mud stone material was found. This was the oldest carboniferous Limestone that we looked at. Some of the fossils found are shown below:

Fossils from left to right

Above: Bivalve shells and two examples of solitary corals

Below: Corals. Those to the left are polygonal corallites or ‘honeycombe’ coral.

Photo below shows the many criss-crossing veins that formed as a result of friction along the thrust plane causing fracturing of the rocks, with the fractures being later in-filled by calcite precipitated from circulating water, and which give the white striped appearance. Stop #5

Beyond the Black Rock limestone we walked across a field which overlay an area of coal measure rocks which were found in the 1800s to have coal seams some 15 to 30cms in thickness. After some excavations, in the more distant past, the amount of coal available was discovered to be not commercially viable. The owner told us the soil here was fairly wet and relatively acid which benefitted from lime “sweetening” for improved grass quality for sheep and cattle. This field (photo below) has distinctive flora in the spring and summer as it has been left chemical free for the last decade. On the edge of the field we saw a prominent ridge of raised rock, which marks the position of the “Ebor Thrust”, formed by Black Rock Limestone. This material was, and still is, believed to be good farm wall building material. In the early summer the ridge of BR limestone is a mass of cowslips. To the left is the site of a pond as the underlying rock of mudstones and coal measures is less permeable than the surrounding limestone.

Stop # 6 Ebor Thrust /coal exploration adit

We next strolled slightly uphill and vaulted according to our age and athletic ability over a couple of styles to encounter a disused coal mine entrance filled with metal rubbish. It had been dug through a horizontal shaft called an “adit” many years ago to see if the postulated coal seam was worth mining. Apparently, the exploration did not show it was commercially viable. Since February Doug and David have dug out and exposed the Black Rock Limestone thrust over top of coal measures. Here you can see the actual fault plane where BRL has been thrust over the top of the coal measures. In the photo you can see the loose block in the middle. The rocks to the left and under are coal measures. To the right are the two layers of limestone. Some pieces are quite light whilst others are darker. Both have veins some recrystalise and go white and have no fossils in them and other blocks retain their dark colour. You can see the where the BRL has been thrust over the top of the coal measures. There is a time gap of 40 million years between the limestone and the coal measure rocks!

Acknowledgements:

My thanks to our host for the day, David, who gave us access to his land and the benefit of his own local Geological knowledge of the area. 2. Many thanks as usual to Doug for patiently correcting errors of omission and commission in my original text. 3. Oxford Dictionary: Geology is full of new terminology; this time another new one for me: “Adit” (From the Latin Adire = to approach.) An adit is a horizontal opening by which a mine is entered or drained.

Walford