deep weathering

preglacial drainage

rocks and relief

Devonian conglomerate, Sumburgh. Granite clasts indicate exposure and erosion of the granite around 380 million years ago

 

 

Uyea, part of The Ness. A marine abrasion platform?

Erosion surfaces

Definition: a land surface of low relief reflecting long term erosion. The surface may now lie well above sea level as a result of uplift or be tilted or deformed as a result of later tectonics.

Several authors have identified surfaces of low relief on Shetland cut across varied geology. Yet the clearest evidence of a former land surface is provided by the hilly sub-Devonian terrain, a surface preserved where Devonian sandstones and conglomerates abut against the basement in both south and north Mainland.

A surface of low relief between 150 and 200 m was described by Coque-Delhuille and Veyret (1988) which bevels the crests of the main ridges of Mainland. This is roughly equivalent to the summit surface of Flinn at 210 m. On rocks of high resistance to weathering and erosion, this fundamental surface is higher, reaching 280 m on the mica schist and phyllites of Saxa Vord. The surface is post-Devonian as it is cut across these rocks. The depth of Early Tertiary erosion on the Shetland Platform probably requires that the surface dates from post-Eocene time.

These authors also recognise raised marine abrasion platforms at around 20 m above sea level. Key localities include NE Unst, The Ness on North Roe and near Sel Ayre on Walls. The existence of such marine platforms would imply higher sea levels than present. Against a background of post glacial submergence, this requires uplift in the late Pliocene or early Pleistocene. The absence of marine sediments, the evident relief of these terraces and the possibility of structural control means, however, that the identification of these platforms as marine in origin remains uncertain.

Flinn (1964 and 1977) identified submerged platforms around Shetland at a number of levels below present sea level (82 metres, 45 metres, 24 metres and possibly nine metres), He suggests that these are remnants of former marine erosion surfaces and the planar form of parts of the offshore zones certainly stands in sharp contrast the Appalachian relief of many parts of the land area.