The cache is placed with the kind permission of the Gloucestershire Wildlife Trust.
Wintour's Leap is so called because Sir John Wintour is alleged to have leapt from the cliffs on his horse to escape pursuing Parliamentarians. Click to read more.
The rock here is Carboniferous Limestone which forms an extensive plateau with a fairly consistent elevation of around 200m. The rock was deposited as marine sediments during the Dinantian Epoch of the Carboniferous Period and was formed between 363 and 325 million years ago.
The weathering of limestone is mainly by solution, a process also known as carbonation. Limestone is made of calcium carbonate. When carbon dioxide from the atmosphere is dissolved in rainwater, it makes a weak acid called carbonic acid. When carbonic acid comes into contact with limestone and passes through joints and bedding planes, it reacts with the rock to form calcium bicarbonate. The calcium bicarbonate is soluble and is carried away in solution, gradually weathering the limestone. As this process continues, the fissures become larger and larger and can eventually result in the formation of extensive cave systems.
In the area where you are standing now, acidic water from the soils has percolated into the joints and widened them to create clints and grikes, a feature of limestone pavements. Limestone pavement is unique to this area in the district and may represent a small part of a much larger area which is now hidden by woodland. Solution hollows created on the clints become eroded over time into strange shapes which often become detached from the bedrock and may be found in rock debris in the subsoil. These strangely shaped rocks are often used as ornaments on stone walls in the Chepstow area.
Solution isn't the only kind of weathering to affect Carboniferous Limestone, especially where the bedding planes are so well defined and relatively shallow as they are here. These bedding planes can be clearly seen if you stand at waypoint 2 (N 51° 39.779 W 002° 39.822) and look at the cliffs to the North. A principle cause of weathering is the freeze-thaw cycle where water enters cracks and fissures then freezes. The water expands when it freezes and breaks the relatively weak bonds between the layers of rock.
To claim this cache, complete the following tasks and email your answers to me via my profile page:
At waypoint 3 (N 51° 39.786 W 002° 39.813), you will see an information sign. In front of the sign, there is an area of flat limestone and roughly 1m in front of the sign there are three or four shallow solution hollows. The one to the right of the sign is the deepest.
1) Estimate, in centimetres, the depth of this hole below the surrounding rock.
2) Assuming an average solution rate of 15 cm every 10000 years, estimate how long the hollow has been in formation.
3) Thinking about the depth of the hole and the varying weather conditions throughout the year, if the hollow was formed entirely by solution, why is your estimate likely to be much too low?
4) The sign gives a reason why Sir John Wintour's survival of the leap was much more likely in those days than it would be today. In terms of the types of weathering mentioned in the text above, what do you think may have been the main cause of the change mentioned on the sign?
Photos of you or your GPS near the cliff would be great but please don't include the sign.
You don't need to wait for my reply before writing your log but please note that logs will be deleted if I don't receive the answers in a reasonable time. All submissions will be acknowledged.