This is the second 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 (this 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. (EarthCache - flash flood deposit). The rock has the splendid name of the Triassic Dolomitic Conglomerate. It can be seen as a building stone in Temple Meads railway station and polished as Draycott ‘Marble’ in Wells Cathedral.
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
From the road at N51°13.828 W002°39.000 walk eastwards along the track through the woods. This last co-ordinate has been changed slightly following a note from the first finder. It should be on the road at the end of the track below Ivy Cottage, ST547481.
Look at the rocks by the sides of the path and on the path. They are composed of reddish, rounded sand (quartz) grains. These are the Devonian rocks (Portishead Formation) and are exposed here in the core of the Pen Hill pericline. They were deposited when the region lay at about 20°S by meandering river systems on a coastal plain lying to the south of the mountains of Wales, the Lake District and Scotland. If you could go back to this time, about 370 million years ago, you would be in a hot, arid desert. Only insects were around on land at this time - dinosaurs, birds and mammals had not yet evolved. There were small trees where there was enough water but not here.
What to do:
Find a piece of unweathered rock and look at it carefully
a. what is its colour?
b. is it made of more than one type of grain?
c. what is the grain size - smaller, bigger or the same as sand size?
Note: If you are collecting small pieces of rock, label this one Devonian, Portishead Formation Old Red Sandstone. A serious collector would also note the location and date.
At N51°13.840 W002°38.815, walk south and, as you do so, you cross the junction of the Devonian rocks with the Jurassic Charmouth Mudstone. Although you don't see a good exposure of the Charmouth Mudstone here, it is easy to imagine the Jurassic seas lapping against the old Devonian shoreline.
Now walk down to New Cut noting a change of rock type to the overlying Blue Lias at N51°13.684 W002°38.746. However, once you arrive at New Cut, it is easy to distinguish the White Lias, N51°13.416 W002°38.784. This is the next EarthCache in this series; - Mendip Hills - White Lias limestone.
Please email your answers to me, JurassicEdie