This is the first 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 (this 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
Turn right from Wookey Hole car park along the High Street, walk SSE to the junction of High Street with Milton Lane. From here follow the footpath SE until you join Lime Kiln Lane. At N51°13.240 W002°39.749 you will see some large lime kilns. Lime kilns were used to produce quicklime through the burning (calcination) of limestone.
CaCo3 (limestone) + heat = CaO (quicklime) +CO2 Layers of lumps of limestone and wood (later, coal) were burnt and the lime raked out from the bottom and riddled to get rid of the ash.
Continue to N51°13.285 W002°39.693. The rock here is the Carboniferous tropical limestone, known locally as the Burrington Oolite. This is a magnificent exposure, the 'gorge' probably having been eroded out along a fault line,
At the start of the Carboniferous, shallow seas spread across the coastal Devonian plain, giving rise to a long period of limestone deposition. The environment in which these rocks formed was similar to the Bahamas, with tropical, warm, clear, shallow water conditions covering the Mendip region. What would it have been like to be there about 340 million years ago? - probably rather wonderful. You would be a little south of the Equator and the sea would have been warm with little bits of sediment wafting back and forth in the currents; these will form the ooids which already cover the sea bed. You may be able to see some brachiopods and crinoids (sea lilies) wafting gracefully in the currents. There may be rugose corals too, as well as plenty of fish. Here you are probably too shallow to see shark but they are swimming in deeper water.
What to do:
1. Find a piece of unweathered rock and look at it carefully
a. what is its colour?
b. how does this differ from the general colour of the whole rock face?
c. what is the grain size - smaller or bigger than sand size?
2. Looking at the whole rock exposure, how do you know the rocks have been folded here?
Note: If you are collecting small pieces of rock, label this one Carboniferous Limestone, Burrington Oolite. A serious collector would also note the location and date.
Walk on to a point near N51° 3.302 W002°39.604 for a good view to the NE to Pen Hill where you may be able to imagine the Pen Hill pericline plunging down towards the west. You are looking at one of the folds (periclines) described earlier.
Continue SE to N51°13.182 W002°39.476. From here turn NE along the path. You are now crossing the Mercia Mudstone, described earlier.
Continue to N51°13.220 W002°39.189 where it will be evident from the rock in the path that you have crossed from the Mercia Mudstone to a different rock type. These are called the Triassic Westbury Formation and Cotham member. They are carbpnate mudstones and are more exciting than they appear here as they represent the gradual flooding by the sea of the red desert plains that existed before. The base of these rocks is amazingly flat in this area, at about 150m, and this flat surface could only have been formed by the sea.
Continue on the path until you join a lane. Go on past Manor Farm (built of White Lias, described later), then turned west at N51°13.423 W002°39.086, crossing the Mercia Mudstone. N51°13.577 W002°39.350 marks the route. Cross the road and walk to N51°13.588 W002°39.301. This is the tropical Carboniferous Limestone again but here it is known as the Black Rock Limestone.
What to do
1. Estimate the dip of the rock here and work out its dip direction
2. Walk a few metres to N51°13.587 W002°39.299. and do the same again
3. What do you think has happened. Please note it is a small-scale structure.
Note: If you are collecting small pieces of rock, label this one Carboniferous Limestone, Black Rock Limestone. A serious collector would also note the location and date.
Walk on to N51°13.743 W002°38.086. From this vantage point you can see a break on the hillside in front of you. This marks a fault line which is the junction of the Carboniferous Limestone with the Devonian desert sandstone and represents the nose of the plunging pericline.
This walk is continued in the next EarthCache: Mendip Hills - desert sandstone.
Please email your answers to me, JurassicEdie