LAKES IN RELATION TO GEOLOGICAL FEATUEES 469 
west face of the Cheviots might be adduced, where Upper Old Red 
Sandstone and Carboniferous strata rest on an ancient platform of 
Silurian and Lower Old Red Sandstone volcanic rocks. South of 
Jedburgh, there are isolated hills carved out of Lower Old Red Sand- 
stone lavas at the close of that period, which are now restored by the 
partial removal of Upper Old Red strata that once enveloped them. 
The evidence now adduced, in brief outline, reveals the extensive 
denudation of the ancient Silurian tableland durinjj; various o-eoloffical 
periods, and the behaviour of the Triassic sediments indicates that 
mesozoic strata entered into the structure of the Southern Uplands 
and of the Midland Valley at the time of the initiation of the river 
systems during the Tertiary period. 
Glaciation of Scotland 
In the preceding section, dealing wqth the evolution of the topog- 
raphy of the country, reference has been made to the fact that in 
pre-glacial time Scotland stood at a higher elevation above the sea- 
level than it does at present. Mining operations in the basins of the 
Forth and Clyde have conclusively shown that coal-seams have been 
worked up to the margins of pre-glacial river-channels now filled 
with various superficial deposits. For example, the bottom of one of 
these ancient river courses at Grangemouth is 240 feet below present 
sea-level. 
But in Scotland no deposits of later Tertiary time have yet been 
detected which might throw light on the changes that preceded the 
advent of the Ice Age. In England, however, valuable evidence is 
supplied by the later Tertiary formations. The older Pliocene 
deposits on the Norfolk coast, which are all of marine origin, were 
laid down at some distance from land, in a warm temperate sea. On 
the other hand, the Newer Pliocene strata indicate a gradual refrigera- 
tion of climate. Professor James Geikie and Mr Clement Reid have 
shown that the land and fresh-water mollusca of the lower part of the 
Red Crag are mainly of South European types, while those of the 
higher zones from the Upper Red Crag to the Weybourn Crag 
present a more northern facies. It is evident, therefore, that during 
these stages the seas of East Anglia must have been connected with the 
Arctic Ocean, and that the North Sea may have been occupied by an 
Arctic fauna. These facts point to the submergence of the Contin- 
ental Shelf and the severance of Britain from Scandinavia across the 
plain of the North Sea. 
The succeeding Cromer Forest Bed, consisting of a series of 
estuarine and lacustrine strata laid dow^n under temperate condi- 
tions, points to a greater extension of land surface than now prevails, 
