Geology

Geology

The valley to the south of Great Elm holds a few geological curiosities – by Jim Duffus.

The De La Beche Unconformity

Firstly, we have the famous De la Beche unconformity in Vallis Vale, which is reached by crossing the bridge at Jackdaws and taking the riverside path for about ½ mile on the left. Here, the angular unconformity is revealed in an old quarry – in simplistic terms, an unconformity occurs when ancient bedrock is forced over at an angle, then eroded until level; subsequent rises in sea level and depositions of sea creatures then results in the upper limestone strata.

De La Beche unconformity (by Muir McKean)

Further evidence of this unconformity is to be seen at the old quarry at Tedbury higher up the valley. Go over the bridge at Jackdaws, turn right, proceed over the metal bridge and bear left along the Fordbury Brook until you reach the bottom of a flight of 37 steps. At the top you arrive at the quarry. It’s amazing because now you are standing on a flat jurassic sea bed (the unconformity). On the far side of the quarry you see the much younger yellowish Jurassic limestones deposited perhaps 150 million years later!
Further information can be found on the British Geology Survey – link here

Elm Cave

Whilst there are a number of cave-like holes along the sides of the Mells River, the one most obvious is Spleenwort Shelter (midway between Jackdaws bridge and Peter’s Bridge. However, the one most interesting and recently rediscovered by Mendip cavers is Elm Cave which is situated between quarry railway line and Tedbury Camp. Mendip Cave Registration indicates exact location – see link. It has to be said that this cave is not easy to find and access to it involves a considerable amount of scrambling.
A few months ago a group of Mendip cavers searched for and rediscovered this cave.
I was fortunate in being asked to come along (for the ride). At this point I would like to say that I used to cave in the Mendip area in the ’70s; now almost 50 years later I am a little less flexible; needless to say the entrance was a bit of a struggle. The chamber slopes down for about 50 feet and then a tight squeeze allows entrance to a second chamber. Fearless by now, I managed the squeeze and reached the bottom of the second chamber where we dropped down to a water sump. Exiting back through the squeeze was an interesting experience; luckily I had a Mendip caver behind me to give a push at the appropriate moment.  Mendip cavers will be issuing a report on this visit to Elm Cave; details to follow. 

Entrance through a manhole (photo courtesy of MCRA)
In the first chamber
A Pitman was there – in 1872?

The Mells Sink

Another geological curiosity is the Mells Sink; to be found just upstream of the Fussell’s Iron works, mid way between Great Elm and Mells.
Essentially, this is a hole in the river bed which acts as a sink or spring depending on the relative heights of river and water table. When water table is high (ie after heavy rain) water flows out into the river, but when water table is low water flows in and tests have proved that emergence is at Hapsford. The Bristol Speleological Society investigated this phenomena between 1974 and 1978 and issued a report. Click here for link to this report of the Mells River Sink.