458 THE FRESH-WATER LOCHS OF SCOTLAND 
From the brief outline already given of the geological history of 
the country, it is evident that the youngest rocks entering into the 
structure of this plateau consist of the Eocene volcanic and plutonic 
rocks of the West Highlands with the accompanying series of basalt 
dykes. Hence the elevation of the plateau must date from that 
period, and the excavation of the valley systems must have been 
effected during the later divisions of Tertiary time. The Mesozoic 
sediments must also have entered largely into the structure of the 
block, though they have been mostly removed by denudation. Much 
of the evidence relating to the topography of the country at the close 
of the prolonged volcanic activity in Eocene time is, as might be 
expected, both fragmentary and indefinite, but the available data 
bearing on the development of the surface features of Scotland can 
be satisfactorily explained on the foregoing hypothesis. 
In Scotland three great planes of denudation can be recognised : — 
(1) the High Plateau or peneplain, varying from 2000 to 3000 feet 
in height, with Ben Nevis, the Cairngorm Mountains, and other peaks 
appearing as monadnocks ; (2) the Intermediate Plateau, the upper 
limit of which is about 1000 feet; (3) the Continental Shelf, extend- 
ing to the 100-fathom line at the edge of the Atlantic Rise and the 
Faroe Channel. Each of these represents a protracted period of denu- 
dation, with the sea acting, in each case, as the base-level of erosion. 
At the initiation of our river systems, Scotland lay mostly, if not 
wholly, on the south-east slope of the uplift, and formed part of a 
continental area continuous with Ireland on the south-west and with 
Scandinavia on the north-east. Across this inclined plateau the 
consequent rivers drained south-eastwards into a Miocene sea stretch- 
ing from the north of France to beyond Schleswig Holstein. The 
north-west declivity of this land surface extended to the edge of the 
Continental Shelf, or, in other words, to the edge of the Atlantic- 
Arctic Ocean Rise, and was therefore steeper and shorter than the 
other. Hence the consequent rivers flowing in a north-westerly 
direction had greater erosive power than those draining to the south- 
east. Thus they were able to entrench themselves in valleys and 
ultimately, by cutting backwards, to encroach on the domain of those 
flowing; to the south-east. 
On the mainland of Scotland subsequent denudation has cut the 
High Plateau into three main blocks, in each of which similar con- 
ditions of drainage occur, viz. : (1) a long drainage system towards 
the south-east along the remains of the old consequent valleys ; 
(2) a shorter and steeper one to the north-west by obsequent streams 
running in the old consequent valleys ; (3) a subsequent easterly and 
westerly drainage system due in large measure to the grain of the rocks. 
These blocks are situated as follows : — 
