The failure was controlled and constrained by a combination of factors:
Joints and fractures within cliff run vertically and parallel to the cliff face enabling wedge-shaped sections of cliff to fall
Coastal erosion and weathering
coastal erosion and weathering of the cliff face are a continual natural process
the sea is eroding the base of the cliff (undercutting), removing support for the rocks above
the processes of weathering weakens the cliff, making it more susceptible to failure
Recent wet weather has added more water into the cliff from above such that grain support is weakened in the Bridport Sand Formation (see Geology below) thereby increasing the likelihood of a landslide occurring.
The rock fall deposit was 30 m long, 20 m wide and 10 m high, which ran out over a gravel beach.
The cliffs at Burton Bradstock mostly comprise the Bridport Sand Formation, a grey, brownish (weathering to a yellow colour), micaceous silt and fine-grained sandstone with stronger sandstone beds occurring irregularly throughout the succession, typically every metre or so. These stronger sandstone beds are more resistant to weathering so protrude from the cliff face as in the photograph (above right).
The Bridport Sand Formation is approximately 180 million years old and is at its thickest in the Burton Bradstock area, forming the 40-metre-high cliffs seen at the coast. It is a weak to moderately strong sandstone that becomes weaker when wet. Most of the landslide deposit was composed of the Bridport Sand Formation with blocks up to 2 m3 in the landslide debris.
Above this is a three-metre bed of limestone from the Inferior Oolite Group that forms a cap at the top of the cliff. This is a stronger material than the Bridport Sand Formation and, consequently, blocks 2 m x 2 m x 0.5 m were seen in the landslide debris.
Above this is a bed of Fuller's Earth, a calcareous mudstone forming shallow mudflows cascading down parts of the cliff.
Contact the Landslide Response Team
British Geological Survey