LAKES IN RELATION TO GEOLOGICAL FEATURES 465 
SCULPTURE OF THE MIDLAND VALLEY 
Although an arbitrary line extending from the Firth of Forth 
to the Firth of Clyde was chosen as the boundary between the 
Central and Southern Blocks, we may here refer to the Midland 
Valley as a whole, as both sides have many features in common. 
This tract, measuring about 1 20 miles in length and about 50 miles 
in breadth, is bounded on the north-west by the Highland fault 
reaching from Stonehaven to the Firth of Clyde, and on the south- 
east by the fracture defining the northern margin of the Southern 
L^plands. 
It has already been shown in the geological section that the 
sediments entering into the structure of the Midland Valley belong 
chiefly to the Old Red Sandstone and Carboniferous formations, with 
which are associated contemporaneous volcanic rocks. The strata 
are arranged in the form of a compound syncline with subsidiary 
minor folds, the longer axes of which are more or less parallel to the 
bounding faults, thus giving rise to a prominent grain of the rocks 
in a north-east and south-west direction. There is ground for 
maintaining that the Midland Valley was originally buried under 
Triassic and younger sediments, which, for the most part, have been 
removed by denudation. 
As soon as the trunk system of drainage had been established at 
the time of greatest elevation, the weak sedimentary strata, attacked 
in flank, soonest gave way, and the system of drainage characterised 
by subsequent streams gained the ascendancy. The volcanic plateaux 
offered greater resistance to the denuding agents, and hence their 
outcrops assumed the form of intervening ridges, while the areas 
occupied by the sediments have been worn down into plains from 
which rise isolated hills and knobs representing major igneous 
intrusions and volcanic necks. These hills of circumdenudation are 
of extreme interest, as they are still breached by the old consequent 
rivers draining the Highland Plateau, and they contain wind-gaps 
indicating the deserted channels of some of these consequent streams. 
Reference has already been made to the behaviour of the Tay, 
North Esk, and Bervie rivers in the north-east portion of the 
Midland Valley. The Forth above Stirling has had a similar history 
to that of the Tay during the period of greatest elevation. It seems 
to have formed an affluent that passed southwards close by St Abb's 
Head to join a stream that drained the Tees, the combined rivers 
flowing north-eastwards across the plain of the North Sea. The old 
buried channels of the Forth and of its tributaries the Bonny, the Devon, 
and the Almond plainly indicate the greater elevation of the land 
during the evolution of the present topographical features. Like 
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