Why are there so many landslides in the Dark Peak?
Travelling along the railway line from Manchester to Sheffield provides an excellent opportunity to see large numbers of landslides along the route.
The Dark Peak is very well endowed with landslides. Alport Castles is thought to be the largest inland mass movement in Britain and became active around 8000 years ago. The slide at Lady Clough is another typical slide-type mass movement and there are many others in the area.
Nearby at Castleton is a famous landslide at Mam Tor (known as 'the shivering mountain') where series of landslides have occurred over at least the last 3000 years. Learn more about Mam Tor and see how it affects man-made features in the area.
How geology affects slopes
The geology of the area is important to understanding the landslides, but there is a complex and interrelated set of reasons why they occur.
- The gritstones are heavily jointed, which makes it easy for blocks to break away and fall.
- Beneath the gritstones lie shales.
These are inherently weaker than the gritstones, especially when they are wet: they are prone to disintegrate in water.
Because water can easily seep through the gritstones, shales may become weakened and the slopes unstable.
The Dark Peak lies within the area of Britain which was glaciated during the last ice age, and which had frozen ground (periglaciation) for some time after the glaciers had retreated.
- Valleys may have become over steepened due to glaciation, making the slopes inherently unstable.
- As glaciers retreated and frozen ground melted the groundwater table rose and groundwater pressures became high.
- Some large movements seem to have occurred around 8 thousand years ago, as this happened - for example those at Alport Castles, Lady Clough & Long Gutter Edge.
Heavy and prolonged rainfall is common in the Peak District.
