Edale Valley, Derbyshire

Derbyshire example

Extensive underground cave system open as a tourist attraction at Winnats Pass in the south of the area.

Geological setting

The upland moorland to the north of the area comprises sandstone and siltstone strata of the Millstone Grit Group of Carboniferous age. These rest on mudstone, sandstone and shale strata, also of Carboniferous age, which are exposed in the valley bottoms and along the southern margin of the sandstone outcrop. To the south of the area the mudstones and shale rests unconformably on massive Carboniferous limestone. The area was not covered by ice during the last glaciation and the landforms of the earlier glaciation have been much modified by periglacial processes during the last ice age. Thus there are only a few areas with a covering of till but large expanses of head. The upland moorland is covered by extensive deposits of peat.

Significant geological hazards

Compressible ground is present where there is a significant thickness of peat, mainly on the high moorland, and in patches of stream alluvium where there is a possibility of layers of peat or interbedded, saturated clay, silt and sand. There is a small possibility of running sand conditions within saturated sand layers within deposits of alluvium along streams. There is a small potential for shrink/swell clay subsidence to be associated with some of the Carboniferous mudstones and Quaternary clay till.

There are several areas where there is a significant potential for landslides hazard to be present which includes areas of currently active landslides (Mam Tor). These areas are particularly associated with steep valley sides formed by thick sandstones resting on impermeable mudstones where a strong springline forms at their junction. The water from the spring line saturates the mudstone and overlying superficial deposits, such as head, lowering their strength and promoting failure of the slope.

The limestone area exposed at the south of the area is well-know for the caves and contains, several of which have been developed as tourist attractions. Although the sudden collapse of natural caves causing subsidence at the surface is an extremely rare event, there is a significant potential for hazards associated with the potential for material to subside into ancient collapse features as their infilling material settles. The possibility of loose material on the surface being washed into subterranean voids under the influence of surface water drainage is a more common cause of surface subsidence.

Landslide on the north side of the Edale Valley.
A6 destroyed and closed by the Mam Tor landslide at the head of the Hope Valley.