We were walking up in the Peak District with Janet and Richard near Sheffield. We were heading to this strange tower somewhere between Manchester and Sheffield. It sits very oddly on its own right next to a cliff face. The picture here doesn't quite capture its strangeness. We thought it may have been a quarry previously - note the strewn boulders. It is known as 'the tower' at Alport Castles.

The Tower at Alport Castles, Peak District. (camera phone picture)
There is a better pictures from above.
Apparantly it is a landslip - possibly the largest in the UK. You can read more about the geology at the Derbyshire Peak District written by MJ Lewis, and how the make up of the rock in the area is unstable and worn by water to create landslides.
Alport Castles lie to the north-west of Ladybower Reservoir, in grid square 1491 of Landranger 110.