This is the fourth of a series of EarthCaches which can be completed on a walk on public footpaths from Wookey Hole to Wells. The whole walk can take 4 hours so refreshment will be needed! Parking is available in Wookey Hole car park N51°13.548 W002°40.318 and there are regular buses from Wells to Wookey Hole.
Background Geology
In this series of EarthCaches, the plan is to look at some of the major rock types that make up the Mendip Hills and to try to imagine what it would have been like to be there at the time. We shall be looking at rocks from about 370 million years ago (Devonian Portishead Formation) through to about 195 million years ago (Jurassic Charmouth Mudstone Formation).
This geological history can be broken into the following four simplified stages:
(1) Movement of 'Britain' i.e. this bit of the Earth's crust from a latitude of about 20°S where conditions here were similar to those of the Kalahari desert today to just south of the Equator where we were in tropical seas with limestone being deposited. The desert rocks are the Devonian Portishead Formation (EarthCache - desert sandstone) and the limestone is Carboniferous Limestone (EarthCache - tropical limestone).
(2) The next stage is created by the movement together of two large tectonic plates which caused much folding and faulting of the earlier Devonian and Carboniferous rocks. The Mendip region lay to the north of the area of the main mountain building, and probably reached an original altitude of about 1500m. If you push a tea towel along a flat surface from one direction, it folds in an asymmetrical way. This is what happened to the rocks in this area. The pressure was basically from south to north with a shortening of the land by about 20km. It gave rise to a series of 'en echelon' folds called periclines. Pen Hill is an example of one such pericline.
(3) By now 'Britain' had moved further north to about 20°N, similar to the Sahara desert today. Once again, we experienced a desert environment. The Mendip region was part of the Pangea supercontinent. The region was a bare upland desert where coarse debris was deposited on upland slopes and in canyons or wadis eroded through the landscape. This debris would often be transported in flash floods, moving material away from the upland areas as alluvial fans, with grain sizes decreasing with distance from the upland areas. Eventually, sediment would be deposited at the mountain front to form a broad apron of debris. Similar conditions can be seen today in upland regions of modern deserts. (this EarthCache - flash flood deposit). The rock has the splendid name of the Triassic Dolomitic Conglomerate.
Apart from reddish soil, the rocks on top of the Dolomitic Conglomerate, called the Mercia Mudstone, are not in evidence on this walk. If you could go back in time, however, the environment here would be similar to the Basin and Range area of the USA but small dinosaurs might decide to investigate your potential for their lunch.
(4) By about 200 million years ago, the sea was gradually coming in over the area to deposit fine mudstones and limestones (EarthCache - Lias limestone) Gradually the Mendip Hills became an archipelago with pterosaurs in the air and dangerous dinosaurs on land.
The route
The road at ST549471 to the waymark can be quite busy. At N51°13.270 W002°38.800 there is a good exposure of the flash flood deposit described in (3) above. It is known as the Triassic Dolomitic Conglomerate and can be seen as a building stone in Temple Meads railway station and polished as Draycott 'Marble' in Wells Cathedral. As you can see it is a jumbled mass of large and small angular fragments. The fact that they are angular and not rounded indicates that they were not transported by the flood water very far.
What to do:
Look carefully at the deposit
a. what colours can you see?
b. what is the range of sizes of the pieces?
Note: It is not possible to collect a sample that is indicative of this rock type. If possible, take a photograph but remember to add a scale of some sort.
To complete the walk to Wells, continue to N51°13.278 W002°38.612 and N51° 3.289 W002°38.525, up the slope to join the A39 at N51° 3.281 W002°38.258.
Walk a short distance uphill and then cross over on to another public footpath. This will lead over a field where there is a good exposure of water-worn Carboniferous Clifton Down tropical limestone at N51°13.175 W002°38.113. Beyond here, walk down a track past a house whose garden is in an old quarry of this limestone, N51°13.074 W002°38.164.
Continue down the road until you walk into the city of Wells.
Please email your answers to me, JurassicEdie