Inherited shore platforms

Significance: a cover of till indicates that there is widespread inheritance of shore platforms from before the last ice sheet

shore platform    inherited coastal forms                                                               Boulder-strewn rock platform emerging from beneath till, Gairsay

On more sheltered shores the shore platform may pass beneath till. If recently uncovered, the platform may carry striations. Examples include St Catherine's Bay on Stronsay, where a rock platform striated SE-NW is overlain by 4 m of till. At Scara Taing, Rousay, a buried platform or platforms above current sea level but within the range of storm waves display crossing striae and are overlain by three separate till units. The widespread burial of shore platforms suggests formation before the last glacial maximum. This is consistent with the preservation at sites on NW Hoy of beach gravels resting on a raised rock platform. Steers (1973) notes that there are extensive shore platforms emerging from beneath till at just above current sea level on the northern shore of Scapa Flow.

The contrast in shore platform development with Shetland is striking. There intertidal platforms are not a major element of the coast. The contrast is partly one of lithology and structure, with the generally flat-lying and often thinly-bedded horizontal sandstones and flagstones of Orkney facilitating the cutting of platforms. The inherited elements of the Orkney coastline suggest however that sea level history is the key factor in explaining this contrast.

Orkney lies close to the zero isobase for postglacial sea level rise, a reflection of the position of the islands towards but not at the periphery of the last Scottish ice sheet. Shetland not only developed its own ice cap, it also has a postglacial history which is entirely one of submergence. Orkney has been close to its present sea level for ~3 ka in the current interglacial. Presumably also in earlier interglacial phases sea level stabilised for long periods at close to its present level, allowing protracted periods of platform cutting.

Wide rock platform emerging from beneath till, Bay of Swartmill, Westray