CHAPTER XVI

DENUDATION OF THE LOWLANDS

THE most cursory examination of the geological structure of the Midland Valley suffices to show that its comparatively level or undulating surface is not due to a horizontal arrangement of the rocks. A section across any part of the district proves that the strata, so far from being flat, are curved and fractured to a remarkable degree. If no change had been wrought upon the surface after the rocks began to bend, we should now find them sinking and rising into broad folds, the bottoms of the basins being sometimes several thousand feet below the tops of the arches. The lines of fault also would be marked by long ranges of vertical precipice, sometimes several thousand feet high. It is not in the least probable, however, that these geological structures ever showed themselves in such a marked way at the surface. The subterranean movements by which the rocks were folded were probably very slow. Hence the arches and fissure-walls, as they rose into the air, would at once be attacked by the denuding agents, and their detritus would be spread over the troughs that were sinking beneath the water. If the rate at which the rocks were bent and fractured and that at which they were wasted chanced to be equal, the upward

folds and lines of dislocation might never make any show at the surface. And even if the rocks were displaced somewhat faster than they could be worn away, the waste was perhaps always rapid enough to make the actual gain of land considerably less in proportion than the total amount of upheaval. So much, at least, is certain, that if ever the folds of the strata rose into the air as wide dome-shaped hills and mountains, they have all been planed down. Millions upon millions of cubic yards of solid stone have thus been worn away, and the surface of the Lowlands has been again reduced to a general uniformity of level.

It is desirable clearly to realise that a vast amount of rock has been worn away from the surface of this region, before we begin to trace out the probable history of the present topographical features. This may best be done by examining a few typical districts and comparing the structure of the rocks with the actual form of the ground.

The Pentland Hills, which now separate the strata of the Midlothian coal-field from those of the western part of the county, were certainly at one time buried under Carboniferous deposits to a depth of probably not less than 5000 or 6000 feet. If the thickness of this covering be estimated at 5280 feet, or one mile, and the hills as fourteen miles long, by three miles broad, the mass of material worn away must have been equal to forty-two cubic miles. This lost portion would surpass by more than five times the bulk of the present Pentland Hills; and, if it could be set down upon the Lowland Valley, it would form a group of mountains nearly a thousand feet higher than the loftiest of the Grampians (Fig. 76).

Again, the valley of the Firth of Tay, as was pointed out in the foregoing chapter, lies on what is in geological structure not a trough, but an arch. Yet not only has the

whole of the top of the arch been worn away, but the work of erosion has gone on until the upward curve of the rocks has been actually hollowed out into the present wide valley. The volcanic ridges which along the edge of the Carse of Gowrie mount up in successive terraces, dipping away to the north-west, once rose high above what is now the Firth of Tay, and arched over till they came down into the Fife Hills. The connecting portion has been removed, but we see the truncated ends of the beds rising up to the southeast from the sides of the Sidlaws on the one hand, and up

FIG. 76.-Section to illustrate the denudation of the Pentland Hills. a, Lower Old Red Sandstone. b, Lower Carboniferous rocks. c, Carboniferous Limestone series. d, Coal-measures. f, Fault.

to the north-west from the Fife slopes on the other. The accompanying woodcut (Fig. 77) is from a sketch taken on the top of Moncrieffe Hill, looking eastward towards the mouth of the Firth. It will be observed that the hills on either side have their long dip-slopes in opposite directions away from the centre of the arch, and present their abrupt escarpments towards each other. The sudden truncation of the beds indicates most impressively that the amount of solid rock which has been removed from this great hollow, if it could be set down upon the plain, would make a range of hills at least as bulky and lofty as the present Ochils.

FIG. 77.-View of the Firth of Tay from Moncrieffe Hill, looking east. (A valley worn out of an arch of the rocks; the long slopes of the

hill-tops on either side show the dip of the

beds.)

Reduced to diagrammatic form the structure represented in Fig. 77 may be expressed as in Fig. 78.

There can be no doubt that the several coal-fields of Scotland were at one time united, if indeed they did not extend continuously across the site of the Southern Uplands into the north.

of England. They have been thrown into folds, the troughs of which now hold the different basins of coal, while from the intervening arches several thousand feet of sedimentary strata have been worn away (see Fig. 76).

I have already indicated that much of this great denudation was accomplished even as far back as Paleozoic time. But the Tertiary basalt dykes, as in the other regions of the country, furnish us with an interesting proof that there has been enormous erosion even within the comparatively brief interval that separates us from the

older Tertiary periods. In the south-west of Lanarkshire and the east of Ayrshire these dykes run along the crests of some of the higher ridges. One of them, which has been already referred to as crossing the Clyde below Crawford and traversing the boundary fault of the Southern Uplands, descends into the valley of the Douglas Water, and then sweeps up along the crest of the Haughshaw and Nutberry Hills at the height of 1712 feet above the sea.1 The vertical distance from the summit to the bottoms of the neighbouring valleys of the Nethan and Douglas Waters is about 1000 feet. It is obvious, as has already been insisted upon, that the present configuration of the surface cannot possibly be that of older Tertiary time. The lava which rose in the fissures

FIG. 78.-Section across the Firth of Tay to illustrate the structure of the ground represented in Fig. 77. aa, Lower Old Red Sandstone covered by and alternating with bb, various volcanic rocks belonging to the same geological period. c, Upper Old Red Sandstone lying on a unconformably. ff, Faults.

never could have reached the tops of the hills, had there been valleys at a lower level ready to tap the ascending column of molten rock and allow it a passage to the surface. Hence we learn that long, wide, and deep valleys have been eroded in the Midland Valley, as well as in the other districts of the country, since the age of the basalt plateaux of the Inner Hebrides, and that the thickness of rock removed cannot have been less, and was probably far more, than 1000 feet.

1 The remarkable persistence of this dyke along the crest of the hills suggests that these heights actually owe their prominence to the presence of the dyke and its influence in retarding denudation.