LOWER PALAEOZOIC FAUNAS 125 



Lower Palaeozoic sediments are actually littoral in 

 part, while in Southern Scotland the varied and in- 

 constant physiography of a volcanic archipelago has 

 left its mark upon the Ordovician and Lower Silurian 

 deposits. 



The geological history of Britain during the " Protero- 

 zoic " era can be summarized as follows. In Cambrian 

 times a shallow sea, with a coast-line on the west, 

 covered most of the district, and progressively extended 

 its range westwards. Along a belt comprising Wales, 

 the Lake District and parts of Southern Scotland, 

 a very great volume of sediment was deposited, perhaps 

 under the influence of a chain of islands (continuous 

 with the Malvern axis) that converted that district 

 into a partly enclosed sea. There was far less detrital 

 matter available for sedimentation over central England. 

 The deposits of the period show a transition from 

 coarse arenaceous strata at the base to a remarkably 

 uniform series of shales at the top. Isostatic depression 

 more than kept pace with deposition, and created a 

 deep but narrow geosynclinal along the central belt. 

 The Ordovician period was marked by a decided reduc- 

 tion in the area of sedimentation, and a striking change 

 in physiography. The rocks of this system seem 

 almost restricted to the Wales-Southern Uplands 

 belt, and include evidence of much contemporaneous 

 volcanic activity. In Silurian times, after local up- 

 heavals, the sea once again spread over most of the 

 British area (the Scottish Highlands excepted), once 

 more receiving the most bulky deposits in Wales 

 and the West of England. Towards the close of the 

 Silurian period, important geographical changes were 

 initiated ; these culminated in the " Caledonian uplift," 

 whereby most of the area was elevated beyond reach 

 of the sea. 



