Carsington Reservoir
Before and After Sections of Carsington Collapse
The slip plane (shown red) occurred along a layer of yellow clay.
The drawing and text below is from: Geotechnical aspects of the Carsington Dam Failure, a paper by Professor A.W SKEMPTON, Department of Civil Engineering, Imperial College, London, UK

Carsington Dam is an earth-fill embankment with a length of 1200 m and a maximum intended height of 37 m above foundation level. It is situated in Derbyshire, in the Midlands of England. On Friday 1 June 1984 the dam was within about l m of its final height. No fill was placed during the weekend, a period of heavy rainfall. Early in the morning of Monday 4 June 1984 a slip started in the upstream bank over a length of 190 m and by Thursday 7 June it extended along a length of nearly 500 m, with a maximum horizontal displacement of 15 m.