Cliff and Beach Erosion
This coastline displays a whole range of features created by erosion, including headlands, cliffs, shore platforms, notches, caves, arches and stacks. The coastline is in the process of dynamic and rapid change, as features are created and destroyed; at the moment there is only one stack, close to Splash Point, and one small arch, below the Seven Sisters.
The most conspicuous landforms are the vertical cliffs and the near-horizontal shore platforms stretching out into the sea in front of them. The shore platform is very visible at low tide, when we can walk out onto it – though it can be very slippery and dangerous.
The process of cliff erosion
The process of cliff erosion. The chalk cliffs retreat in a series of vertical collapses. These are aided by vertical fissures which open from the cliff top and may for a long time be filled (and hidden) by sediment. The cliff top is often not as solid as it looks, so walkers should keep well back from the cliff edge. When the fissures open up, especially in winter after rain and frost, an entire vertical sheet of rock suddenly peels away and collapses, leaving the cliff up to a metre further back. The vertical collapse is often triggered by wave action eroding a notch at the high water mark at the foot of the cliff; this leaves the face of the cliff unsupported. The erosion is mainly controlled by networks of natural vertical and horizontal fractures within the chalk. Often the rock collapses by sliding over a diagonal fracture, and this produces a buttress. Several buttresses can be seen along the Seven Sisters.
Formation of wave-cut notches
1. Time and weather weaken the top of the cliff.
2. The sea attacks the base of the cliff forming a wave-cut notch.
3. The notch increases in size causing the cliff to collapse along existing fissures.
4. The backwash carries the rubble towards the sea forming a wave-cut platform.
5. The process repeats and the cliff continues to retreat.
What is Chalk?
Chalk is the one rock everybody can recognize. Chalk is easily identified by its pure whiteness. In terms of minerals, it is an unusually pure rock, more than 99% calcium carbonate. Chalk has been quarried for hundreds of years for lime to lighten the heavy acidic soils of the Weald, and for cement-making. It has been quarried for thousands of years for the seams of flint found within it. Flint was used in the stone age for making tools, like the 15 neolithic axes found in Seaford High Street in 1986. The Seaford Axe Hoard is now on display in Seaford Museum. Did you know?: Blackboard chalk is not chalk rock, but made instead from calcium sulphate powder.
How did the Chalk form?
Chalk is formed from lime mud, which accumulates on the sea floor under sub-tropical clear-water conditions. This is then transformed into rock by geological processes: as more sediment builds up on top, and as the sea floor subsides, the lime mud is subjected to heat and pressure which removes the water and compacts the sediment into rock. If chalk is subject to further heat and pressure it becomes marble. The lime mud is formed from the microscopic skeletons of plankton, which rain down on the sea floor from the sunlit waters above. The chalk in our cliffs formed on the seabed during the Cretaceous period, between 89 and 85 million years ago.