Mam Tor is composed of horizontal layers of shale and millstone grit. Shale is a soft flakey rock originally formed by the mud from rivers gently settling on a sea bed. (This area was a warm shallow sea about 350 million years ago) Millstone grit is formed in the same way, but when the rivers are flowing more vigorously and dump coarser stones and grit on the sea bed.
Gritstone and shale stacked on top of each other make the hillside unstable as rainwaters flows between the layers and erodes the softer shale away until the rocks above become unstable. The last ice age left the slope too steep and about 4000 years ago the first slope failure happened leaving a spectacular 80m high scar. Movement continues at about 25cm per year on average, but is far greater after prolonged heavy rain.
Around 200 years ago the main Sheffield to Manchester road was built across the slope to benefit from the lower gradient than the old road up Winnats Pass and was subsequently rebuilt many times, but following a large movement in 1977 the road was finally closed in 1979. It now provides graphic evidence of the movement of the shale.
To claim this cache email us via the profile with the following information
1: The name of the company that built the road, it is on the signpost by the turning zone at the base of the hill.
2: The current mean movement of the slide mass (annual)
3: Also please log a photo of yourself, with your GPS at the site showing the slippage with you to give scale (optional)