466 THE FUESH-WATER LOCHS OF SCOTLAND 
the other east-flowing streams, the Forth occupied the ground first, 
and, by working along the weak belts, captured the old consequent 
rivers of the Central Block as far west as Loch Long. Subsequently 
the streams working inwards from the west have regained part of 
this drainage area. 
Striking examples of wind-gaps are represented by Glen Farg and 
Glen Eagles in the Ochils, the Endrick-Carron hollow across the 
Campsie Fells, and the Blane-Glazert depression between the Campsie 
and Kilpatrick Hills. 
On the south of the Midland Valley the Pentland Hills form 
another ridge of circumdenudation. The course of the river Lyne, 
which traverses this ridge, furnishes remarkable evidence of the 
former existence of topographical features that have long since 
vanished. Rising on the north side of this chain, where it has been 
beheaded by tributaries of the Water of Leith, it still flows through 
these hills as a consequent stream, maintains the old course across 
the West Linton plain, and enters the Southern Uplands in a 
matured valley. lu this chain there are two additional instances of 
old consequent rivers, the North Esk and its tributary the Glencorse 
Water, which, beginning on the northern slope of these hills, cross 
them in deep valleys. On emerging from the ridge of Lower Old Red 
Sandstone volcanic rocks, the Glencorse Water enters the plain 
occupied by Carboniferous strata where it has been captured by 
the Esk and made tributary to the Forth. 
Brief allusion may now be made to the probable development of 
the western drainage of the Midland Valley. The trunk river flowing 
along the course of the North Channel, by working its way backwards 
across the mesozoic strata spread over the plain now occupied by 
the Firth of Clyde, captured the old consequent streams up to that 
now represented by the lower part of Loch F'yne. By following the 
weak strata of the L^pper Old Red Sandstone and the Cementstone 
Group beneath the Carboniferous volcanic rocks of Renfrewshire, it 
deflected the old drainage system of the Cowal region and the heights 
near Loch Lomond, which, for a time, flowed eastwards to the Forth. 
Beyond this point it probably was aided in its recession by taking 
advantage of one of the hollows established by a tributary of the 
Forth. On reaching the Clyde basin above Dalmuir it captured the 
lower portion of the river Clyde, which, as an obsequent stream, had 
for a time discharged its waters into the Forth. 
SCULPTURE OF THE SOUTHERN BLOCK (SOUTHERN UPLANDs) 
The Southern Block, as already indicated, has a core of Silurian 
strata, with a persistent north-east and south-west strike, pierced by 
large igneous masses, and more or less surrounded by less resistant 
