In this account Sir James Hall describes early observations  around Edinburgh. We recognise now much of what he describes as relating to glacial landforms produced by the great ice sheets that have flowed across Edinburgh but Sir James interprets the features in terms of the biblical flood.



On the Revolutions of the Earth's Surface


By Sir JAMES  HALL,  Bart. Pr. R. S. ED. & F.R.S. LOND.

PART II. {Read June 8. l812.)             Transactions of the Royal Society of Edinburgh, 1812, vol VII, 169-211.

In attempting to apply these principles to the great scale of geology, and to vindicate my opinions on this curious subject, I shall appeal to a series of facts which are very accessible to this assembly, the greatest part of them lying within two or three miles of this city. In that view, I have given, along with this paper, a plan, on actual survey, taken on this account, of a small district in the neighbourhood of Edinburgh, comprehending the Corstorphine Hill and its immediate neighbourhood.  I thus hope to indicate the place of each specimen alluded to, in such a manner, that, provided the rock remains in existence, it may be in the power of an observer to discover it at any future period; however much it may have been concealed by those accidents to which such specimens are perpetually exposed.

The country in the neighbourhood of Edinburgh is what all are agreed to call Secondary, consisting of beds of sandstone, and occasionally of limestone; and coal, interstratified with thick assemblages of shale, in loose and frail strata: This mass is traversed with the utmost irregularity, by dikes or veins of whinstone, which occur also in vast interjected masses, sometimes lying in great amorphous blocks, and sometimes in thick beds, parallel to the strata. The strata, themselves, as might be expected, are thrown, by means of its intrusion, into much irregularity, and though nearly parallel to each other in any particular spot, exhibit the utmost variety, when different places are compared together, as to their dip and direction.  This contrast is conspicuous in Salisbury Craig on the east and in Corstorphine Hill on the west of Edinburgh. Each consists of a thick mass of whinstone, parallel to the strata beneath it, which, in the first mentioned hill, dip rapidly to the east, and in the second to the west.

The surface of this district, together with the alluvial part of its mass, bears every mark of the effects which a wave of sufficient magnitude to overwhelm it, might be expected to occasion upon so multifarious an assemblage.

Raised from below by the violent and abrupt means already alluded to, in my last communication to the Society, this district would present to a stream overwhelming it at any subsequent period, numberless points of attack. Many of the rocks being rent in various ways, the hardest parts being in a shivered state, would easily be carried forward.  The soft beds of shale or of slate‑clay being laid open to the attacks of the current, would be deeply abraded by its action, and thus masses, both stratified and unstratified, that were originally unbroken, would be undermined, and, yielding to their own weight, would add to the quantity of moving matter, and extend the field of attack upon the weaker parts. The water would thus be loaded with a multitude of blocks of every size, shape, and quality, and with a quantity of clay, which being soon reduced to mud, through which these stones were irregularly and confusedly scattered, would flow at the bottom of the water, and along with it, and would be deposited, according to the laws already pointed out, when the stream approached to a state of rest. Such seems to have been the origin of that body of compact blue clay which forms a material part of our low districts, bearing every indication of having flowed as a mass into its present situation; for it is totally devoid of stratification, though frequently of great thickness.

This mass shows itself in several places, in the bed of the Water of Leith, where the banks have been laid open by natural or artificial means.  It was well displayed formerly, and may still be seen above the Bells' Mills Quarry, and is now exposed to view upon the right hand, after crossing the Dean Bridge, on the old road to Queensferry. It here presents to view a face of about twenty or thirty feet deep, though it often extends to forty or fifty feet. We find it also upon the shore to the west of Leith, as laid open by the sea; and I am informed by the person who conducted the work, that at the Fort in that neighbourhood, in a search for water, it has been penetrated to the depth of eighty feet from the surface, being fifty below high‑water mark. It is obvious, that the power of such mud, when flowing as a stream, in transporting heavy bodies, and in abrading assemblages previously formed, must bear some relation to the resistance which it would oppose to any object forced through it, and of course, that the existence of assemblages of this sort, affords, by its simple testimony, a powerful argument in favour of a stream having overflowed this country, superior in magnitude to any known river; and the facts seem to meet the challenge held out by Mr PLAYFAIR in the following passage, Illustrations, art. 366. "Lastly," he says, "if there were anywhere a hill, or any large mass composed of broken and shapeless stones, thrown together like rubbish, and neither worked into gravel, nor disposed with any regularity, we must ascribe it to some other cause than the ordinary detritus and wasting of the land. This, however, has never yet occurred, and it seems best to wait till the phenomenon is observed before we seek for the explanation of it."

Now it appears to me, that these vast assemblages, in which blocks of every size, and shape, and quality, some sharp, some round, are confusedly scattered through clay, are inexplicable by any diurnal cause, and do call for some particular solution.

Such parts of the torrent as encountered less of the strata of shale and clay, would hurry along with them the comminuted sandstone, and deposit it in the form of sand and of gravel. Vast accumulations have thus been formed in all our lower districts, the external figure of which, and of the clay, has acquired, as we shall soon have occasion to point out, a character peculiar to itself, and having externally a smoothness and regularity, which forms a striking contrast with the  abrupt and most irregular dislocation which very commonly occurs in the solid mass within.

In the midst of this general wreck of all the frail parts, the strongest masses, principally those which, like Arthur's Seat, have been powerfully pervaded with whinstone, would resist and defy all the impetuosity of the stream. The principles which we have endeavoured to lay down, as to the influence of firm obstacles on depositions and abrasions, would thus be brought into action.

The rock upon which the Castle of Edinburgh stands, together with the site of the Old Town, exhibit the most perfect example that could be conceived of the application of these principles. The rock itself, about two hundred feet in height, above its base, and bare on three fourths of its circumference, consists of one of the most complete and uniform masses of whin stone that is known in this country. Its form is rudely cylindrical, and from it the ridge upon which the Old Town stands, composed partly of deposits, and partly of protected strata, extends, gently sloping, for about a mile to the eastward, from the Castle to the Abbey of Holyroodhouse, where the tail terminates. Round the western, southern, and northern sides, a hollow valley occurs, which, towards the north, is still a marsh, and was once a lake, being known by the name of the North Loch.

Corstorphine Hill, which, as seen from Edinburgh, occupies the horizon to the north‑west, affords, in one respect, an example of the other case just mentioned. It consists of a ridge of about a mile and a half in extent, rising in the middle, declining gently at both ends, and pointing from north to south, with a declination of about 20˚ to the east.  It presents a smooth face of whinstone to the west, towards which the mass dips in parallelism with the strata beneath it. Upon its eastern side a hollow valley occurs, in which the old castle of Craig‑crook stands, and from its southern extremity a tail extends to the eastward, lying between Ravelstone and Murrayfield, upon the southern face of which Murrayfield stands.

Thus, the Castle of Edinburgh, gives an example of the effect of a narrow obstacle; and this hill, of a broad one, in so far as it has a valley on the side towards which the stream was flowing.

We have endeavoured, in the last communication to this Society, to account for the formation of such lakes as occur in alpine and rocky districts. The circumstances just pointed out, explain the formation of those which belong to districts formed of frail and moveable substances.  At Lochend a striking example occurs, of a lake produced upon the upper side of an obstacle, in consequence of local acceleration.

Immediately on the east of Corstorphine Hill, a set of firm rocks, or little hills of sandstone occur, rising up from this hollow, or standing upon its eastern side. Of these, Ravelstone, Craigleith, and Blackcraig, are the principal, well known as excellent quarries. From each of them a tail or prolongation extends to the eastward, formed chiefly of the blue clay already mentioned, together with beds of sand and gravel.  These decline very gently, and maintain, to a considerable distance, the individual character given to each by the firm mass producing it.  These ridges, however, are occasionally interrupted by the interference of the same principles; as we see well illustrated near the rock of Craigleith Quarry, by which the tail extending from the Maiden Craig, (another sandstone mass to the westward of it,) is abruptly cut off.

From Corstorphine Hill to the eastward, the country embracing all the space between Edinburgh and the sea presents one continued series of ridges, upon one of which the New Town of Edinburgh stands. It is an important circumstance, that these ridges maintain a very correct parallelism with each other, with the tail of the Castle Rock, and of the Calton Hill, and with the alluvial prolongations that extend to the eastward from all the eminences of this neighbourhood. And a series of parallel ridges occur also on the south side of Edinburgh, extending from all the rocky eminences, as may be well seen on the road leading to Dalkeith, which passes over several of these; one of the most remarkable of which is, that on which the village of Libberton stands.

Such an arrangement cannot have been the work of the diurnal waters produced by our common rains; for the course of such waters, flowing by the action of gravity, and guided by the general slope of the country, which declines towards the Firth of Forth, ought to have produced depositions nearly at right angles to those under consideration. It is in vain that a vast duration is ascribed to the influence of an agent, unless it can be shown, that its action has a tendency to produce the alleged result. If it has a tendency to produce a different result, that difference would be augmented in proportion to the duration of the action.  Now, the diurnal operations are everywhere found in the act of corroding and altering the forms here alluded to*; but they are nowhere seen to produce them. This class of facts, on the other hand, all conspire in giving probability to the hypothesis of a diluvian wave,  which affords an easy explanation of all the large features of this country.

An important principle of the theory of running streams must here also be considered, namely, that the shape assumed by such a stream flowing through sand or other loose matter, bears a distinct relation to the magnitude of the stream; the radius of curvature of its bendings being in proportion to that magnitude. Thus, all the water collected from this neighbourhood, is capable of producing no more than a paltry brook, as appears from the Water of Leith, which we see meandering between two of the ridges just mentioned. This meandering course, suits the diminutive size of the brook; whereas these ridges being straight, or, mathematically speaking, having a curvature whose radius is of infinite length, we are led, by a very obvious analogy, and in concurrence with what has been observed in other parts of the globe, to believe that a cause very different from any now in activity, and far more powerful, has exerted its influence upon this spot; that a stream has flowed over it, capable of overwhelming and disregarding objects by which the Nile or the Ganges would have been turned out of their course.

But the testimonies in favour of this hypothesis are not derived from these large features alone; and it is not conceivable, that such agents could have been at work, without leaving behind them indications of their influence still more unequivocal. These occur in the very places indicated by the theory, and exhibit remarkable instances of abrasion. In order to investigate them with success, we must have recourse once more to the effects produced by one of our common streams.

Where a firm rock of any kind has been exposed to the action of a rapid river, its surface acquires in consequence of that abrasion a peculiar character, which every one recognizes at a glance, but which is difficult to describe in words.  The most obvious and universal effect of such an action, is the rounding of all the original angles of the rock not only the prominent, but also the entering angles.  For where an original hollow has occurred, coinciding at all with the course of the stream, the water has undergone an acceleration along that hollow, and has excavated for itself a waving groove more or less longitudinal.  The whole has thus acquired a peculiar character, by an assemblage of flowing lines, which recalls the water‑worn state of the rock.  Another set of forms also present themselves in all such cases, which could not easily have been foreseen, and whose existence we learn only from observation of the fact: we observe, that the surface is in many parts excavated by shallow depressions of various sizes, which I shall distinguish by the name of scoopings, as resembling the effect which would be produced upon a soft body by the oblique blow of a spoon or scoop. I conceive that they have in fact been produced by the action of eddies of inferior force to the main stream, but acting in company with it in different and sometimes opposite directions. These various corrosions going on together, have each produced its peculiar effect; and most of them being concave, their meeting has given rise to the set of waving angular ridges which constitute the most unequivocal feature of a water‑worn rock. These angular forms differ completely from those which occur in the broken surface of a rock. These last are acute, rectilinear and abrupt; while those others are continuous, flowing, and having their angles very obtuse; so obtuse in some cases, as not to be visible, unless the light strikes upon the rock in a peculiar direction *.

In a small but rapid brook near my house in the country, these forms occur on the surface of a smooth bed of sandstone over which it flows. I observed lately, when the brook was low and clear, that, to a certain distance below each of these obtuse angled ridges, the rock was covered with green moss, while above the angle it was bare. The mode in which these forms are produced and maintained, seems thus to be pointed out: the main stream being possessed of just power enough to keep the rock clear of moss, and the eddy being too weak for this purpose. In a flood, I presume that the eddy acquires such power, that the whole rock is cleared.

In whatever manner we account for the production of these forms, it is certain, that they present themselves on the surface of water‑worn rocks.  I have also observed them upon the surface of a mass of snow which had been acted upon, and partly removed by a strong wind. As the abrasion occasioned by a fluid in motion seems alone to possess that power, we may reasonably conclude, then, that a fluid has acted where such forms occur.

Now, it is a fact of which I shall presently lay various examples before this assembly, that the very rocks over which, according to the theory advanced in this paper, torrents of water have flowed, loaded with sand, and gravel, and large stones, and accompanied with streams of mud, are found to exhibit at their surface all the characters of abrasion lately mentioned; ‑the rotundity and flowing character; ‑the excavation of hollows into the form of waving grooves; ‑the concave scoopings, and the obtuse‑angled, and waving ridges.  Circumstances, then, seem to justify the conclusion, that, in fact, mighty torrents have traversed these districts.

In addition to the facts mentioned, and in company with them, another set also occur in these scenes, which powerfully corroborate the same conclusion. As stones of considerable bulk are often carried down by torrents, it is reasonable to expect, that upon rocks along which they have been hurried, and on which they could not fail to act as grinders, traces should show themselves of that passage by scratches and abrasions of various sorts; and I have little doubt, that such will be found, when. the effects of great floods in rapid rivers are properly examined; especially where a  stream of mud has accompanied the torrent.  Not having had occasion, however, to visit any scene of this sort, since the importance of the observation  occurred to me, I have as yet, only met in rivers with cases in which the surface of the rock has been dressed to smoothness, and in which the abrasion shows itself in the general rotundity of the grooves and scoopings, and in, the obtuse angled and waving ridges just described *.

But what I have hitherto looked for in vain in common rivers, occurs universally in the diluvian scenes, where there is reason, from other circumstances, to believe, that a powerful abrasion has taken place, and where the surface has been protected from the injuries of the weather. Where it has been exposed to that injury, we generally find, that the large features of dressing, the grooves and scoopings, and obtuse angled ridges, only remain. But where a mass of this kind, either by accident or design, has been followed under ground to where its surface has been protected by a covering of clay, an interesting and striking scene presents itself; the surface is found to resemble that of a wet road, along which a number of heavy and irregular bodies have been recently dragged; indicating that every block that passed, and every one of its corners, had left its trace behind it; and these are rendered very distinctly visible, when the surface, is drenched with water.

In many cases these furrows or scratches have been so deep as to resist all the effects of the weather, and show themselves in rocks that have been always exposed, sometimes many yards in length. Occasionally, single scratches, and parallel sets of them, deviate by five or six degrees from the general direction; but the important circumstance is, that such deviation is rare, the very great majority of both sets agreeing in parallelism with each other, and with the general direction, not only of the scoops and grooves of the rock upon which they occur, but also of the ridges and large features of the district. A rock covered with these furrows, has externally an appearance greatly resembling what is called Slickenside, with this difference; that in the slickenside, we can always discover some proof that one portion of the main rock has performed a small slide upon the other; whereas, in this case, everything shows, that the rock under consideration has stood firm, and has been abraded by a number of bodies in motion. The circumstance just mentioned, of occasional deviations of parallelism, seems also to distinguish this form from the slickenside; in which last, I believe, the lines are invariably parallel*.

The direction of the stream in the neighbourhood of Edinburgh, as indicated by the medium result of a number of observations, appears to have been from 10˚E S. of W. to 10E N. of E., by true bearings taken with a needle, and allowing 27˚E west of north as the variation; and I have met with no case deviating more than 10˚E or 12˚E from that average on either side.