Abraded shore platform at Thorntonloch
Pothole on the shore at Thorntonloch
Car Rocks, Seacliff, East Lothian, during a north-easterly storm around the time of the Spring Tide at the vernal equinox on 30/03/2010. Waves are breaking on the outer edge of the shore platform and wave water is surging across the mid-platform area. Waves then reform towards the rear of the platform before breaking on the backing cliff. Maximum water depth is ~3m. Further footage is available for The Gegan
Undercutting and collapse of a gently dipping sandstone cap rock at Cove
Worm bore holes in limestone
Shore platform processes
The remarkable Carboniferous weathering pits re-exposed by erosion at current sea level at Catcraig. The original pits were a product of solution on the exposed surface of a tropical coral reef around 320 million years ago: some retain a fill of mudstone.
What are the processes which together lower and extend shore platforms?
Shore platforms are exposed to a wide range of processes of weathering and erosion that act to lower and extend the platforms. Conventionally, these processes are grouped conveniently as wave action, subaerial weathering and biogenic weathering and erosion but many processes interact. Moreover the intensity of each set of processes varies not only across platforms but also between sites on different lithology.
Wave action relates to the fracturing of rock on the platform, the detachment of blocks and the transport of debris across the shore platform. That waves are capable of rock fracture on shore platforms is demonstrated by the presence of fresh scars on steps, including the small cliff at the edge of platforms in resistant rocks at the low water mark. Most erosion however may relate to the detachment of already fractured blocks along joints and bedding planes. This operates at many scales, from the removal of tiles of mudstone to the movement of foundered metre-sized blocks of cap rocks
A measure of wave power is provided by the size of boulders that lie on the shore platforms. Some platforms or areas of platform are largely free of blocks and backed by steep storm beaches - here wave power is sufficient to drag even large blocks to the rear of the shore platform. More generally, however, the shore platforms of East Lothian retain a litter of blocks. Some are glacial erratics, reworked from former cover of till whilst most are released when cap rocks are undermined and collapse. Here the maximum size of block that has been moved during recent and historic storms can be judged from the largest clasts found in the storm beaches at the rear of the platform compared to those that remain on the platform. Where boulders rest on soft mudstones or tuffs differential weathering may leave the larger boulders sitting on pedestals.
Where debris is transported to gravel beaches at the rear of the shore platform then this material will be mobilised in storms and lead to abrasion of the rear of the platform. This may lead to the development of notches at the base of the cliff. Occasional rock falls are testament to continuing cliff retreat.
Subaerial weathering may include a wide range of chemical, physical and biological processes. The higher parts of sandstone and limestone platforms commonly show blackened surfaces, a result partly of lichen growth and partly of the migration of Mn and Fe in solution to the rock surface. Weathering appears to be particularly effective at the water layer, with pedestal rocks confined to the zone on platforms where the rock is exposed to the air at low tide. This water layer weathering relates to a combination of wetting and drying and the effects of salt corrosion and perhaps crystal growth. Frost may affect any exposed part of the shore platform surface on winter days.
Biogenic weathering also takes many forms. A wide range of marine organisms bore holes in rocks, especially limestone. These include molluscs, polychaete worms, sponges and barnacles. Micro-organisms may also produce organic acids that weather rock surfaces. Seaweed may act to protect rock surfaces on the lower shore but equally the waving fronds may produce a sail effect during storms, leading to detachment of blocks.