1 92 1. No. II. 



THE STR.WDFLAT AND ISOSTASY. 



Fig. 163. The Nab south of Lerwick, the west side of Bressay iti the background. 



the western sea-board of Unst consist chiefly of micaceous and horn- 

 blendic gneiss with limestones and quartzites. In Unst and Fetlar. the 

 north-eastern islands, there are large masses of serpentine and gabbro. 

 In Delting and Northmavine, in the north-western portion of Mainland 

 a large area is occupied by diorite. The islands Foula, far out to sea 

 toward the south-west, and Bressay, east of Lerwick, are built up of the 

 lower Old Red Sandstone, and so is the greater part of Walls, in the 

 western portion of Mainland. 



As these rocks, building up the islands, ha\'e to a great extent con- 

 siderably less power of resistance to erosion than the rocks of the west 

 coast of Norway in the same latitude, it is natural that the islands have 

 an aspect very different from the Norwegian coast. They are on the whole 

 low with rounded forms, while the shores are cut back by recent marine 

 erosion, forming cliffs and many isolated vertical rocks, the so-called 

 "drongs", in the sea outside. 



But in spite of these conspicuous marks of a recent shore erosion, 

 no ([uite convincing evidence of the existence of a real emerged strandfiat 

 could be discovered along the shores. Many low islands occur, and low 

 flat plains extend inland from the shore in many places in Mainland. 

 But nowhere did I ol)serve a sharplv defined boundary between the coastal 

 plains and the hills rising above them. 



It seems to me, however, to be probable that the low land, about 

 20 to 30 metres above sea-level, extending along the southern east coast 

 of Mainland, northwards from Sumburgh Head (see Fig. 162, I — IV) is 

 actually an old strandfiat. Inside this low land the hills rise more steeply, 

 but as a rule not abruptly with a sharply marked boundary. In some 

 places especially on the points, there may be a more marked difference 

 between the flat shore land and the rising hill side (cf. Fig. 161, \s the 

 distant low point on the west side of the sound north of Lerwick, see also 



