Thor's Cave from the Manifold Valley
There is a convenient and free car park alongside the River Manifold near Wetton Mill (at N53° 06.118, W1° 51.598). From there, walk south to the bridge to Wetton Mill Tea Rooms to see the river flowing underneath you.
Now follow the tarmac track south along the valley, listening to the river alongside you. Vehicles may be using this road, so watch out for traffic. (If you find yourself climbing up a hill, you've gone the wrong way!) You may prefer to use the old road further to your right, by crossing the ford or using the footbridge.
The limestones in this valley (which are almost pure Calcium Carbonate) were formed in warm tropical lagoons some 325 to 360 million years ago when the land that is now Britain was close to the equator. (Look up Plate Tectonics in Wikipedia for a full explanation of how the Earth's surface is divided into 'plates' that collide with each other to create earthquakes and mountains.) The seas teemed with life and brachiopods, corals and crinoids all flourished. The rocks were formed by the deposition of layers of sediment, mostly shell debris and mud, that were later compacted and/or cemented into hard beds of limestone and thin beds of shale. The layered sequence also also contains lens-shaped masses of reef limestone that lack obvious layering. The reefs form prominent features of the valley as they have resisted the later erosion caused in the last 2 million years by the glacial melt water during the Ice Ages.
Limestones are soluble in rainwater (which absorbs Carbon Dioxide from the atmosphere as it falls, to become a very weak carbonic acid) and typically dissolve rather than wear away. They are also well jointed and highly permeable allowing water to pass through. Water percolating underground slowly widens these joints to form extensive undergorund cave systems which develop best in reef limestones rather than the surrounding thinly bedded limestones as the reef limestones are physically stronger and do not collapse as the caves form.
Rivers in this landscape respond rapidly to rainfall variations, which also affect the level of the groundwater. In dry weather the water sinks into the ground via "swallets" and the surface river runs dry. But during wet weather, the water often bursts out of the river bed via resurgences as the underground passages fill quickly. The river then runs on the surface.
Q1: As you walk down the road, please estimate the coordinates at the point where the river stops flowing along the river bed. Note you should not leave the roadside when doing this.Add this information to your log with a note of the weather conditions. Your readings will help build records of the river's behaviour.
Now carry on down the road to N53° 05.578 W1° 51.261 where an information board gives more details of how Thor's Cave has been used by man and animals over the ages. Take the footbridge to your left, and climb the limestone steps to follow the path until you see a signpost to Thor's Cave. Fork right here and continue climbing until you reach Thor's Cave itself. Look at the smoothly rounded sides of this Karstic cave.
Q2: Click on my name at the top of this page to send me an email, telling me what you think created this cave. (Hint: do you think it was eroded by temperature, glaciers, wind, water or man?) What has caused the colours on the sides of the cave?
If you now look north up the valley, you will see Nan Tor cave
Q3: In your email, tell me why you think these two caves are at a similar height and alignment. What would the rocks between them have been like before the present valley was eroded?
To log your visit, please give details of where the river sinks into the ground, and email me with the answers to questions 2 and 3. Logs that do not have an accompanying email will have to be deleted.
Further information about this location is given in The Hamps and Manifold Geotrail leaflet, freely available from local information offices, libraries and visitor centres. It can also be downloaded from http://srigs.staffs-ecology.org.uk/Geotrails/HampsManifold/hampsmap.pdf
Well done barlowsaurus for logging this cache first!!!