erosion surfaces

deep weathering

rocks and relief

 

Preglacial drainage

The glaciation of Shetland has disrupted but not obliterated the pre-established drainage pattern. This is partly a reflection of a way in which ice sheet flow has often been transverse to the trend of the main valleys, although the topographic control over ice flow was greater at times of thin ice cover. The trend of preglacial valleys can be established where remnants of such valleys survive as benches above the steep sides of glacial valleys and fjords. The reconstruction points to a pattern of drainage similar to that of the present, with radial drainage towards St Magnus Bay and the Fetlar basin. The valleys were already incised before glaciation, suggesting uplift of Shetland in the late Neogene. It should be possible to trace the valley systems across the platforms east and west of Shetland and into the main sediment receiving areas of the Viking Graben and the continental shelf.

A different approach to drainage reconstruction is to examine the patterns of deposition offshore. The Palaeocene sands of the northern North Sea were accumulated in a series of overlapping fans. Debris was transported by large rivers which drained the Orkney-Shetland Platform and flowed to the southeast. The garnet composition of these sediments implies erosion of former Permo-Triassic cover rocks and so the river systems may originally have been superimposed on the basement (Morton et al., 2004). The Quarff gap may be a fragment of a Devonian valley that carried sand and gravel from mountains in the west to what is now the Cunningsburgh area.