Geology Reference
In-Depth Information
13
COASTAL LANDSCAPES
The relentless buffeting of coasts by waves and their perpetual washing by currents fashion a thin line of unique
landforms. This chapter covers:
waves, tides, and currents
cliffs, caves, and other erosional coastal landforms
beaches, barriers, and other depositional coastal landforms
humans and coasts
Cliff retreat: the Beachy Head rockfall
Beachy Head and the Seven Sisters cliffs to the west are made of a hard chalk and stand up to 160 m above sea level
along the southern coast of England. On 10 and 11 January 1999, the upper part of Beachy Head collapsed. The
fallen portion was about 70 m long and 17 m deep with a mass of 50,000-100,000 tonnes, which buried the cliff base
and is seen as 5-10 m of accumulated chalk rubble with blocks up to4mindiameter. The failure occurred along one
of the vertical joints or minor fault planes that are common in the chalk. To the east of the main slip, a fracture some
2 m deep and 1 m wide and extending up to 10 m from the cliff edge has appeared. It runs down most of the cliff
face. The toe of the chalk rubble is eroded at high tide, when it produces a sediment plume that runs from just east of
Beachy Head Cave to the unmanned Beachy Head Lighthouse. The cause of the rockfall is not certain, but 1998 was
a wetter year than normal, and during the fortnight before the fall heavy rain fell on most days. The wet conditions
may have increased the pore pressures in the chalk and triggered the rockfall. Events such as the Beachy Head rockfall
have been occurring for thousands of years along southern and eastern English coastlines and have led to cliff retreat.
