bathymetry

coastal erosion

fjord

 

The making of the Shetland archipelago

The island group that is Shetland is a landmass drowned by postglacial sea level rise. The individual islands have been separated one from another by a range of geological and geomorphological processes.

The Permo-Triassic basins around Shetland sit between structural highs. The Fetlar Basin is rimmed by the much older crystalline rocks of Skerries and Fetlar and St Magnus Bay has a similar ancient origin. It may be that Permian sediments once were more widespread and have been stripped from other small sedimentary basins around Shetland. The first control on the pattern of islands is thus structural.

It is very difficult to reconstruct a preglacial drainage pattern on Shetland but the division of the islands into hill masses may have begun when rivers flowing to the Atlantic and North Sea excavated valleys between them. The steep drop to water depths of 80-100 m around many parts of the outer coast of Shetland certainly encouraged linear glacial erosion and the cutting of the deep valleys that now form the firths. The alignment of these drowned valleys is generally controlled by faults and bands of weaker rocks. During periods of maximum ice cover the ice probably flowed across the main N-S valleys but these valleys guided ice flow at times of restricted ice volume. Smaller islands may have been isolated from their neighbours by marine erosion but the limited duration of current sea level requires that such erosion belongs to the last interglacial or earlier.