LAKES IN RELATION TO GEOLOGICAL FEATURES 463 
Middle Old Red Sandstone had probably the greatest development 
among the younger formations in the Central Block. Indeed, the 
slope of the country from the High Plateau to the Moray Firth is 
partly due to the resuscitation of the old floor of crystalline schists 
on which they were laid down. The apparently abnormal direction 
of the shore-line from Fort George to Kinnaird Head, bounding the 
present plain, is probably due to the strike of the planes of uncon- 
formability in that region, at the base of the Trias, at the base of 
the Upper and of the Middle Old Red Sandstone. 
The drainage towards the Moray Firth was probably established 
at an early date. The disposition of the Old Red Sandstone and 
younger strata and the trend of the shatter belts favoured the action 
of the subsequent streams, so that the successive divides were broken 
down and the disjointed members of. the old consequent rivers were 
made tributaries of the subsequent system of drainage. The Spey is 
the most striking example, for it seems to have been able to cut back- 
wards so as to intercept the old consequent streams of the Northern 
Block as far west as l>och Eil, though subsequently compelled to 
yield part of this ground to invaders from the north and west, 
especially to tributaries of the Great Glen. 
The plain in Eastern Aberdeenshire is evidently due to the 
removal of Old Red Sandstone and probably mesozoic strata. The 
trunk river that received the drainage of the greater portion of this 
low-lying tract ran parallel to the existing eastern coast-line of the 
Northern Block, and intercepted all the consequent streams probably 
as far south as the Tay. The Ugie, the Ythan, and the Ury are 
remnants of old consequent streams that crossed this plain. 
The Dee valley has been determined largely by the grain of a 
belt of crystalline schists lying between large masses of granite on 
either side. The history of the Upper Dee and Don suggests that 
these rivers were the first to occupy that portion of the High Plateau. 
For it has been clearly shown that the Feshie captured the head- 
waters of the Geldie, and that the Avon beheaded the Upper Don at 
Inchrory, thus deflecting the drainage at these points towards the 
Spey. The Tilt — a tributary of the Tay — has also pirated the Tarf 
from the Dee. 
The behaviour of the rivers in that part of the Central Block 
lying to the south of the Highland Border fault between Stonehaven 
and the Firth of Clyde has been studied and described by several 
observers in recent years. Brief allusion may here be made to some 
of the salient points and their bearing on the evolution of the topog- 
raphy of that region. 
The Lower Old Red Sandstone along this belt consists of a 
succession of conglomerates, sandstones, and marls with intercalated 
