CHAPTER XV

PHYSICAL FEATURES AND GEOLOGICAL STRUCTURE

BETWEEN the southern flank of the Highlands and the northern edge of the uplands of the pastoral counties, lies that wide hilly depression which, for want of a better name, I have been accustomed to call the Midland Valley. It is only in the broad sense, as a band of lower ground between two ranges of high land that it can be spoken of as a valley ; nor can a district so plentifully dotted with hills, and even traversed by long chains of heights, be in strictness termed a plain. Looked at from the geological point of view, however, this belt of lowland country occupies a broad depression defined by parallel dislocations, and bounded on either side by hills which consist of older rocks. It may thus be regarded as a valley, owing its form and direction to geological structure.

Between the other two great belts of Scotland and the central zone, there is the fundamental distinction that they owe their prominence to having been upraised relatively to the surrounding areas, while the intervening tract has undergone subsidence. In the Highlands and Southern Uplands the oldest rocks in the geological structure of Scotland are exposed. In the intervening Lowlands, younger formations

which once spread out over the hilly regions on either side, but have since been almost wholly removed thence, have been preserved by having been let down into the hollow. The results of denudation accordingly present themselves here in another aspect.

In order to follow intelligently the progress of evolution in the development of the scenery of central Scotland, it is needful to attend with some care to the character and thickness of the various geological formations that occupy the area, to the manner in which these are now arranged with regard to each other, and to the probable conditions in which they were successfully deposited.

Reverting for a moment to the geological structure of the Highlands and Southern Uplands, the reader will remember that the great terrestrial movements which affected these regions before the Upper Silurian period, threw the deep masses of older Palæozoic sediments into vast plications, ruptured them with innumerable faults, pushed them over each other, tore up vast masses of the underlying platform of Archæan gneiss, and so crushed these and the Paleozoic strata as to give rise to a newer series of gneisses and schists. He will recollect also that these prodigious disturbances in the crust of the earth were directed in a general sense from south-east to north-west, that consequently the trend of the dislocations and of the axes of plication runs from southwest to north-east; and that it is this structure which has determined the prevalent north-easterly course of the great belts of country in Scotland. To these subterranean movements, and to others subsequently continued in the same average direction, the Midland Valley owes its north-easterly trend, and the straight parallel sides which bound it.

If we could clear out all the younger Palæozoic deposits that now fill up the broad central depression of Scotland, we

should probably find the Lower Silurian rocks of the Southern Uplands continued in endless anticlinal and synclinal folds towards the Highland border, becoming gradually more metamorphosed in that direction, and eventually passing into the grits, conglomerates, and slates of Callander, Aberfoyle, and the edge of the Highlands. But though continuous underneath, they may never have formed a continuous tract of hilly ground at the surface. On the contrary, there is good reason to believe that the very movements which plicated, fractured, and upheaved the areas to the north and south depressed this central region. And if this be true, then it would follow that the Midland Valley had its general position determined as far back as Palæozoic time.

The oldest strata anywhere exposed in the centre of Scotland, between the margins of the Highlands and of the Southern Uplands, belong to the Upper Silurian period. They are only seen in two or three limited areas, where they come to the surface from under the vast pile of sedimentary accumulations which elsewhere overlie them. In Lanarkshire, and the southern part of the Pentland Hills, they attain a thickness of probably not less than 3500 feet, but their base is nowhere seen, so that we do not know what their total depth may be. From their gentle inclination and gradual passage upward into the great overlying Old Red Sandstone in Lanarkshire, they were evidently deposited after the time of the chief disturbances that convulsed the Lower Silurian rocks. But their occasional crumpling and plication show that these disturbances had not finally ceased.

Next comes the Lower Old Red Sandstone-a pile of conglomerates, sandstones, shales, and volcanic rocks, attaining a total thickness in Kincardineshire of more than 20,000 feet. This vast mass of sedimentary and igneous material appears to have accumulated in a lake that spread over

the Midland Valley, and probably stretched south-westward into the north of Ireland, and north-eastward across what is now the floor of the North Sea. The lake, I believe, owed its origin to the subterranean movements just referred to. It was an area of depression bounded on the north-west and south-east by tracts that were undergoing uplift. The energy of the underground forces is well shown in the thick masses of lava and volcanic ashes that were discharged from numerous volcanoes along the floor of the lake, and now form important ranges of hills. In the Pentland Hills these accumulations are at least 5000 feet thick, and in the Ochil Hills they exceed 6000 feet.

Originally, of course, the Lower Old Red Sandstone covered the whole of the Midland Valley. But in the course of long ages of geological change, its superficial area has been restricted, chiefly owing to the spread of younger formations above it. Yet it still occupies wide belts of country, and forms the highest and most picturesque ground in central Scotland. It runs along the northern side of the Lowlands from the coast of Kincardineshire and Forfarshire south-westward across the island to the Firth of Clyde, where it is well seen in Arran. Along this northern belt, its associated volcanic sheets form the long chain of the Sidlaw and Ochil Hills. I have already referred to the evidence that the Old Red Sandstone of this district once extended northwards into the Highlands. No one can walk along the conglomerate hills between Callander and Crieff, without recognising how strong is the proof that such was the case (ante, p. 141, and Figs. 29, 30).

There is no corresponding continuous broad belt of Lower Old Red Sandstone along the southern borders of the Midland Valley. The same kind of rocks, however, appear there in a tract of moory heights stretching from the