The scratched and grooved surface of the boulder was produced when it was fast frozen in some iceberg, and driven gratingly across some submarine summit, or stranded on some rocky coast-line. But, from its rounded form, the stone had evidently undergone a long process of wear and tear previous to its glacial journey. Probably it had hitherto lain along a surf-beaten beach, where in the course of ages it had gradually been worn into its present rounded shape. But how came it there? It must originally have formed part of a flat sandstone bed, with many other beds piled above it. By what agency, then, was this great pile reduced to fragments?
The answer to these questions must be a somewhat lengthened one, for the subject relates not to a few beds of rock hastily broken up and dispersed, but to the physical changes of an entire country, carried on during a vast succession of geological periods.
A phenomenon, known familiarly as "crag and tail," has long been connected in its origin with the drift or boulder beds. Has my reader ever travelled through central Scotland? If so, he must often have noticed the abrupt isolated form of many of the hills, presenting a mural front to the west, and a long sloping declivity to the east. From the great number of isolated hard trap-rocks in this region, the phenomenon is much better seen than in most other parts of the kingdom. There is, for instance, the castle rock of Stirling, with its beetling crag and castellated summit, which present so imposing a front to the west. Many other examples are seen along the line of the Edinburgh and Glasgow Railway. The range of hills south of Linlithgow, the singularly abrupt basalt of Binny Craig, the long rounded ridge of Ratho, the double-peaked crag of Dalmahoy, the broad undulation of woody Corstorphine, are all examples more or less marked. Edinburgh itself is an excellent illustration. The Calton Hill shows a steep front to the town, while its eastern side slopes away down to the sea. Arthur's Seat, in like manner, has a precipitous western face, and a gentle declivity eastward. The Castle rock, too, shoots up perpendicularly from the valley that girdles it on the north, west, and south, sinking away to the east in a long slope—
"Whose ridgy back heaves to the sky,
Piled deep and massy, close and high."
East-Lothian presents several well-marked instances; in particular, North Berwick Law and Traprain. A phenomenon so general must have had some general origin, and it was accordingly attributed to the same agency which produced the drift-clays and the striated rock-surfaces, when these were believed to be the results of great diluvial action. It would seem, however, that the phenomenon of crag and tail should not be associated with the boulder-clay. The latter is undoubtedly a newer Tertiary formation,[10] but the denudation[11] which produced crag and tail must have been going on long ere the Tertiary ages had begun. There is satisfactory evidence that large areas of our country were planed down at a greatly more ancient period than that of even the oldest of the Tertiary series. Thus, the whole area of the county of Sussex suffered a very extensive denudation during the later Secondary ages. The Hebrides had undergone a similar process previous to the deposition of the Lias and Oolite, and the Greywacke hills of south Scotland, previous to the formation of the Old Red Sandstone. There seems thus to have been a general and continuous process of degradation at work during a long succession of geological ages.
[10] The reader is referred to the table of the geological formations at the end of the volume for the relative position of the beds described.
[11] Denudation is a geological term used to denote the removal of rock by the wasting action of water, whereby the underlying mineral masses are denuded or laid bare.
The results of this long-continued action are of the most startling kind. I have referred to the phenomenon of crag and tail as perhaps the most readily observable. We must not fail to remember that the crag which now stands up so prominently above the level of the surrounding country, at one period lay buried beneath an accumulation of sandstone, shale, or other strata, all of which have been carried away, so as to leave the harder rock in bold relief, with a portion of the less coherent strata sloping as a long tail from its eastern side. The crag, too, is often breached in many places, worn down at one end, rounded on the summit, and sometimes well-nigh ground away altogether, whilst in front there is invariably a deep hollow scooped out by the current when arrested by the abrupt cliff. In [Fig. 2, a] represents a crag of greenstone worn away and bared of the shales which once covered it; b, the sloping "tail" of softer strata, protected from abrasion by the resistance of the trap-rock, and covered by a deep layer of drift, d; c marks the hollow on the west side of the crag.
