process

rates of erosion

raised platforms

inheritance

Key sites

Belhaven

Chapel Point

The Gegan

 

Shore platform cut across dipping ORS, east of Siccar Point

 

 

 

 

 

 

 

 

 

 

 

 Platform morphology on dipping sandstone at Seacliff. Note the blackening and weathering of the highest beds and the undermining and collapse of the more competent beds.

 

 

 

 

 

 

 

 

The King's Kist, Aberlady. Remnant of the raised shore platform isolated by cliff retreat

 

 

 

 

 

 

 

 

 

Basalt erratic resting on a pedestal at Scoughall

 

Shore platform morphology

The shore platforms in East Lothian are gently inclined towards the low water mark and it is often unclear if the platforms extend below this. On more resistant lithologies there is a rock step at low water spring levels but on weaker rocks the seaward edge is buried beneath rubble or hidden by seaweed. The platforms are wide, ranging in the outer Firth from 100 m to a maximum of 900 m at Tyne Mouth and in the inner Firth west of North Berwick from 50 to 200 m. The widest platforms are eroded into weaker sedimentary rocks with low dips, including the Carboniferous Calciferous Sandstone and bedded tuffs.

The shore platforms are backed by 15-25 m high cliffs at Dunglass, Dunbar and Tantallon but the high cliffs at St Abbs Head show only narrow shore platforms. More generally, the rear of the platform terminates against a low cliff cut in rock above which lies a raised shore platform on which the Main Postglacial Raised Shoreline rests.

The relief on the shore platform is a reflection of the local geology. Dips along this coast are often low and so flat platforms are favoured. Where dips increase and there are lateral variation in rock hardness then differential weathering and erosion leads to the development of cuestas with a height of up to 2 m. Erosion is concentrated along the strike, where undermining by washing out of weak rocks leads to undermining and collapse. On the volcanic rocks, vent agglomerates and basaltic sills and dykes form upstanding stumps, walls and scarps whereas tuffs tend to form low areas on the platforms. Rock mass strength is an important control on platform morphology.

On sections of coast with large exposures of competent sandstones or agglomerate, as at Cove, Scoughall and Partan Craig, two levels are evident on the shore platform. The upper level is covered only at high tide and appears to be largely inactive at present. The lower inter-tidal level lies 1-2 m below and is undergoing active erosion, leading to undercutting of the margins of the upper level. At The Vaults, Dunbar, and the King's Kist, Aberlady, remnants of the raised platform beneath the raised beach remain as stumps above the inter-tidal platform.

The shore of Gosford Bay and at Scoughall Rocks is littered with glacial erratics, washed out of a former till cover. Most of the erratics are large sub-angular to sub-rounded basalt boulders. Similar erratic boulders are common on more sheltered platforms. Since these large blocks can only be moved by exceptionally powerful seas, the boulders tend to remain more or less in place. Long-term stability is indicated by the presence of boulders on pedestals. Other large boulders of local lithology are a result of the collapse of cap rocks on the platform during lowering - these generally lie close to where they fell.