largest landslip in Europe.
physical causes: mountain composed of alternating permeable sandstones and
impermeable shales which dip slightly towards valley.
shales (finely bedded mudstones) can be crumbled easily so little strength.
upper layer gets slippery when wet and provides lubricated layer for rock above to
slip down over it.
about 3000 years ago water saturated the sandstone beds adding weight until the
stress was too much that the rock mass broke along and slid down a rotational
concave slip plane. left a 70m high slip face with a typical arcuate shape and uptilted toe at its foot. many slips since and they still occur.
most significant one was a hazard in 1979 when major road between Manchester
and Sheffield which crossed the slip toe and had been repaired many times was cut
and abandoned as was no longer economic to repair it. heavy vehicles have to take
a long detour. each layer of the exposed remains of the road represents a repair a
repair after a slip. 5 are visible and in places the repairs are 2 metres thick. broken
shale can be seen where the material is still slipping aided by spring which issues
where the brown vegetation is. wet shale is slippery, so layers above easily slide
over wet layers beneath. in the face of the scar, composed of thin beds of
permeable rock, small mudflows indicate where the wet shale has turned back to
the mud it once was.
below the toe is debris consisting of material from the landslip which threatens to
bury farm buildings in its path. landslip and flow together are more than 1km long.
average flow movement is 250mm per year but faster in wetter years. continuously
moved by up to 500mm per year since its formation.
