464 THE FRESH-WATER LOCHS OF SCOTLAND 
volcanic rocks, the latter being typically developed in the Ochils and 
Sidlaw Hills, where they form a well-marked arch. Between this 
outer range and the Highland Border, the overlying sedimentary 
strata, composed of sandstones and marls, lie in a trough now 
represented by Strathmore and the Howe of the Mearns. The 
Upper Old Red Sandstone rests with a strong unconformability 
on the lower division of that formation, outliers of it being found 
at intervals on the coast of Forfarshire and Kincardineshire. In 
this region, as in the Pentland Hills, there is clear evidence for 
maintaining that the members of the Lower Old Red Sandstone 
were folded and denuded before the strata of the upper division 
were deposited. 
A glance at a geological map shows that the rivers Tay, North 
Esk, and Bervie traverse the marginal belt of the Highlands in deep 
consequent valleys, thence cross the plain occupied by the Lower 
Old Red conglomerates, sandstones, and marls, and breach the volcanic 
arch of the Ochils and Sidlaw Hills. Ultimately they joined the 
trunk river that flowed northwards along the East Coast. It is 
obvious that, at the time of the initiation of these consequent streams, 
Strathmore and the Howe of the Mearns had no existence. There 
must have been a graded slope from the margin of the Highlands 
towards the south-east. The behaviour of the rivers on entering the 
belt of weak strata along Strathmore reveals the processes by which 
the existing topographical features were brought about. Thus the 
Isla, a subsequent tributary of the Tay, by working north-eastwards 
along the weaker Lower Old Red strata, has captured several of 
the old consequent streams draining the Highland Plateau from 
the Ericht to the Upper Isla. The deflection of these waters 
into the Tay led to the initiation of obsequent streams draining 
into the Isla on the north-west slope of the Sidlaw Hills, and the 
formation of wind-gaps across the volcanic arch, of which the 
hollow traversed by the Dundee and Alyth Junction Railway is a 
good example. 
In like manner the South Esk, which may be regarded as a 
subsequent tributary of the North Esk, by working south-westwards 
along the same weak strata has tapped the old consequent streams 
of the Highland Plateau as far to the south-west as the Prosen. 
Again, the Luther Water, a subsequent branch of the North Esk 
from the north-east, has captured several minor consequent rivers. 
Wind-gaps resulting from this deflection are still to be found in 
that portion of the Sidlaw range ; one conspicuous example occur- 
ring to the east of Marykirk. Similar phenomena on a smaller 
scale are observable where the Bervie River crosses the Howe of 
the Mearns. 
