moorlands such as lie on the confines of the counties of Lanark and Ayr. But over the low grounds, they are for the most part buried under a smooth-swept covering of drift, so that any minor features which they might have lent to the landscape are concealed. It is in such confined spaces as ravines, or along the sea-margin, that we can best learn after what fashion the softer rocks of the Lowlands yield to the attacks of time.

The disappearance of the older stratified deposits under a widespread mantle of drift leads us to consider how far the scenery of the Midland Valley has been modified by the ice-sheets and glaciers of the long Ice Age. This subject will form the theme of the next chapter.

CHAPTER XVII

THE GLACIATION OF THE LOWLANDS

WHEN the great ice-sheet began to settle down upon Scotland, the main features of the broad Midland Valley, like those of the rest of the country, were probably very much what they are still. During the passing of the Ice Age many of the minor details of the scenery were modified or obliterated; hills were rounded and smoothed, while many an old valley and river-course was partially or wholly filled up with boulder-clay. The whole country was, as it were, smothered up in drift, and hundreds of new hillocks and mounds were scattered over its surface. Yet the larger elements of the landscape underwent no marked change.

Throughout the Lowlands, no chain of hills seems to have been high and broad enough to nourish an independent group of glaciers. But, as we have already seen, the great ice-sheets from the Highlands, on the one side, and from the Southern Uplands on the other, streamed down into the low grounds and across these to the sea. Hence the projecting rocks have that ice-worn surface so characteristic of Scottish scenery. They still retain abundantly on their sides and summits the striation and moulding which mark the direction in which the ice moved. From

the position and trend of these markings, we learn that the massive ice of the great Highland area came down into Strathmore and kept steadily southward in such force as to mount over the chain of the Sidlaw and Ochil Hills, and to have been massive enough to envelop and bury the Pentland Hills, for striæ have been found on the top of Allermuir Hill, at the north end of that chain, at a height of 1617 feet above the sea. Hence the stream of ice that passed eastwards across the Lothians must have been a good deal more than 1600 feet thick. It has left its mark on every prominent hill and crag in the district. Some excellent examples of ice-striæ may be seen on the side of the Queen's Drive round Arthur's Seat, just above the crag of Samson's Ribs, where the ice has forced itself through a narrow gully on the south side of the hill. The north side of North Berwick Law retains some remarkably fresh ice-groovings which show that hill to have been enveloped and ground down by the ice in its eastward progress. Farther west a huge body of ice descended from the north into the basin of the Clyde, filling the firth, overriding the hills on either side, and passing across the site of Renfrewshire and the north of Ayrshire and Lanarkshire until it met the ice that was streaming northward from the Southern Uplands into the plains of Ayrshire, whence the united glacier turned southward into St. Patrick's Channel, and spread out over the basin of the Irish Sea. The general trend of the various branches of the ice-sheet, in their course across the Lowlands, will be most intelligibly followed by an examination of the accompanying Map of the Glaciation of Scotland. The reader can there trace the march of the different ice-streams that poured into the Midland Valley and, uniting their mass, moved eastward and westward into the sea. With so vast a thickness of ice creep

ing over the land it is no wonder that there should be roches moutonnées even on the tops of many of the Lowland ridges, and that the rocks should show such widespread proofs of having had their roughness rounded off.

I have left for fuller consideration in this chapter the results of the abrasion of the surface of the country by the ice-sheet, as manifested in the abundant detritus that has been left in the valleys and all over the lower grounds. Before the Ice Age began, Britain had probably stood for a long time above the sea. Its superficial rocks had consequently been exposed to protracted sub-aërial disintegration. Much of the decomposed rock would of course be washed off by rain. But over the flatter surfaces of ground, where the transporting power of rain would be least, the rotted rock probably accumulated in situ. In the south of England, which the ice-sheet did not reach, a considerable thickness of such decomposed material may now be seen, representing probably a prolonged period of sub-aërial disintegration. When the ice-sheet settled down upon the country, and began to creep over its surface, this superficial layer of rotted rock would first be removed. The ice thus found a vast quantity of loose material ready to be pushed onward and to be made into various kinds of 'drift.'

Of all the varieties of detritus left behind by the ice, the most universal and characteristic is the till or boulderclay. This interesting deposit may be seen exposed in banks and cliffs along the course of almost any Lowland brook, from the sea-shores up to a height of 1700 feet or more. Its maximum thickness is reached in the Carse of Stirling, where it attains a depth of 160 feet.

The boulder-clay has had much influence on the character of the scenery of the Lowlands. As its surface is for the most part smooth and undulating, it gives an aspect of

gentleness and tameness to the landscape that would probably be greatly lessened if the clay could be stripped off so as to show the bare rock underneath. The long, smooth boulder-clay slopes, which have been already described in the account of the surface of the Highlands and Southern Uplands, are more marked and more abundant in the Lowlands where the deposit spreads over so much larger an area. In like manner, the water-courses which have so generally been eroded through the clay, present almost everywhere sinuous grassy bluffs, here and there cut into

fresh scars as the streams attack them, and as springs cause landslips along their fronts.

But the most marked surface-feature presented by the boulder-clay of the Lowlands is its tendency to assume the form of long ridges ranged parallel with the general trend of the striæ on the rocks below, as described in Chapter XIV. They may be seen all over central Scotland. In the Lothians, for instance, they run nearly east and west. Hence roads which follow that direction may continue for