Evolution of a sea stack in Oregon

Orkney stacks

Shetland stacks

sea level change

 

The blade-like profile of a stack defined by major joint sets and on which rock fall continues in response to erosion of the cliff base

Stacks of Duncansby

Significance: an imposing set of pyramidal or pillar-like stacks developed in horizontally-bedded sandstones isolated by cliff retreat

The Stacks of Duncansby are some of the most impressive in the British Isles. The Great Stack is over 60 m high and rises above the summit of the adjacent cliff. The stacks rises above a narrow shore platform and its partially-submerged reefs along a stretch of shoreline where cliff retreat appears to have been significantly more marked than to the north at Duncansby Head and to the south beyond Fast Geo to Skirza Head (Crampton ad Carruthers, 1914). The pyramids and pillars (or "cletts") cannot simply be coastal forms exhumed from a till cover as these delicate forms are unlikely to have survived the passage of glacier ice, although more substantial rock bosses may have done. The tallest stack now lies around ~200 m from the cliff edge and this gives a maximum estimate of the amount of postglacial marine erosion. More generally, stacks in Caithness are <100 m from the backing cliffs. As sea level has been close to sea level only during the last 6000 years then maximum rates of retreat are of the order of 33-16 mm/yr but probably vary widely over short distances. A minimum rate of retreat is perhaps indicated by the width of the narrow platform just above present sea level at the base of these cliffs. If this platform relates to the main postglacial shoreline then erosion over the past 6000 years has failed to remove it.