The Seaton and Beer cliffs show magnificent exposures of the most westerly chalk cliffs in southern Britain. The Cretaceous deposits lie unconformably on Triassic red Mercia Mudstones in the west of our area and on Jurassic Blue Lias towards Lyme Regis in the east. This is due to the generally easterly dipping pre-Jurassic rocks at the time of the later deposition.
The Mercia Mudstones, the red colour is due to the oxidised iron mineral haematite, were laid down as lake muds approximately 235million years ago. Then about 100million years ago the sea encroached on the area initially depositing sands, the Upper Greensand, and then in some areas lime muds which became limestones, such as the Beer Head limestone. After this came the deposition of the chalk, as billions of algal coccoliths formed a sediment on the sea floor.
Later, during the Tertiary period, major earth movements took place. Folding and faulting occurred
and the sea floor was uplifted. After this there was erosion of the upper levels
and deep valleys were carved through the Chalk and Upper Greensand to expose the
mudstones.
During these earth movements a major fault formed running from Seaton Hole inland, causing the western side to drop relative to the east. This is clearly shown where the Cretaceous greensand and chalk of White Cliff, at Seaton Hole, lies alongside the mudstone cliffs of the West Walk of Seaton promenade.
Beer, 1.5 miles west of Seaton, is a fishing village nestling in a small combe. It's bay is surrounded
by chalk cliffs. The chalk and limestone layers show a pronounced dip centred on
Beer due to the earth movements in the Tertiary. Within these cliffs to the west
are quarried the famous Beer freestone. This limestone, occurring at the base of
the Middle Chalk, has been quarried since Roman times and has been used in many
major buildings including, St. Paul's cathedral and Exeter cathedral.
East of Seaton, is found the famous Dowlands Landslip. This is one of the largest landslips ever recorded in Britain. It occurred on Christmas Day in 1839 and aroused great interest, with even Queen Victoria coming to view the site from her yacht in the bay.
A popular explanation of the landslip is as follows: the Cretaceous rocks are
relatively permeable and water filtered down finding an impermeable block when
it reached the Triassic below. The water lubricated this plane of unconformity
enabling slippage to take place. This was accentuated when earlier rotational
cliff falls, which had provided a protective toe of rocks to support the cliff
face, gradually eroded due to wave action. This caused tension, well inland,
which on the fateful night gave way as the vast block of the Cretaceous rocks
glided seawards, allowing a deep chasm to form behind it. The movement also
caused an upthrust offshore, in the compression zone, to form a lagoon. This new
reef was eventually eroded away by the sea. (see sketches below)
Geoff Marshall
