The History of Stirlingshire
Chapter XXXI – Geological Landmarks

Subsequent to the irruption of the trap into the coal measures, we have many evidences in the Falkirk district of the existence of what is called by geologists the drift, or boulder-clay flood, and this appears to have been one of the chief operative agents in giving the land which has been left us its present configuration. The reality of such a flood was clearly shown by Sir James Hall, and no one can doubt its existence and power who will look at the well-known phenomenon – the "Crag-and-tail," so well seen at the castles of Stirling, Edinburgh, and other places, where the solid rock has protected the softer stratifications lying on the south-east side. The natural inference from this is, that the flood set in with its chief force from the north-west, and keeping this in view, let us consider what would be its effect upon the adjacent country when the land, as it evidently then did, stood at a much lower level than at present; and to do this the more effectually let a position be taken on the high ground – on Cannel Moss, for example, to the south of Falkirk, which is now about 612 feet above the level of the sea. But before commencing this survey, it may be well to notice how peculiarly, even now, this portion of Scotland is situated. Take, for instance, the neck of country through which the Forth and Clyde Canal runs, and we shall find that, its highest point is under 150 feet above our present sea level. If a canal were therefore dug, only 20 fathoms deep, in the same line, we should have a direct communication between the Atlantic Ocean and the German Sea. The extraordinary effects that were produced by this great flood must force themselves upon the mind of any one who will look at the huge rolled boulders that are exposed in almost every field to the south of Falkirk, or have been excavated in the neighbourhood, and these generally from fragments of rock not otherwise found in the district. In the railway cutting between Laurieston and Redding, some large boulders of grey granite were found, portions of which were afterwards converted into elegant curling stones. At the same place, or at Brighton’s quarry, above the sandstone may be seen multitudes of boulders, several many tons in weight, embodied in the stiff clay, or "till" as it is sometimes called.

From the elevated position we are supposed to occupy, let us now look towards the west and north-west – the directions from which the flood has come. We perceive at once that there are two great gullies, or valleys, through which this flood must have chiefly set – the one between the Denny hills and other high grounds on the south, and the Ochils on the north, with the trap rock of Stirling castle in the centre; and the other between the Denny hills on the north, and the high ground west from Cannel Moss on the south. These two great currents would naturally meet a few miles to the east of Falkirk, and the more northerly one seems to have struck with great force the land which now forms Bo’ness Bay, and probably was the chief agent in scooping it out. The south current has, apparently, been less intense, most likely from the protection afforded by the Denny hills; but even here we shall find that its effects have been very decided, both upon the strata carried away and those which have been left. Directing our view still further north, we see that it was in all likelihood the same agent which scooped out what must, at one time, have been a bay at the Bridge of Allan; for here, also, the current, no doubt, flowed with great force, striving to get an exit through the Stirling valley.

Generally speaking, it is found that when the currents have been strongest, there the denudation of the strata has been greatest, and a knowledge of this fact might be advantageous to landlords and coal proprietors in their searches for the different kinds of minerals. The survey already taken will show this pretty accurately, for at Bridge of Allan – the most exposed of the localities mentioned – all the upper portions of the stratification have been swept away, and consequently we find not the coal measures of the Falkirk district, but the old red sandstone. At Bo’ness, on which the chief force of the north current seems to have been reflected, only the lowest portion of the coal deposit is formed; while at Bannockburn appears the coarse sandstone which underlies the Shieldhill coal-field. On the south side of the Denny Hills, and between the two currents at Kinnaird and Grangemouth, we find some of the higher of the Shieldhill coal strata. At Bonnyside again, which was specially exposed to the influence of the south and less powerful current, all the upper Shieldhill minerals have been removed, and those above the lower Bannockburn series are alone left; while from Glenfuir eastwards only the lower and inferior series of the Shieldhill minerals have been, or are ever likely to be, found. In fact, from the elevated position occupied, it seems to us a simple matter to say generally where coal ought to be got, and where it need scarcely be looked for – the important consideration being as to whether it was protected or otherwise from the operation of the flood. Depth beneath the sea bottom, elevation above the sea level, and the shelter afforded by some solid mass of rock, seem to have been the main circumstances by which this valuable mineral was locally preserved.

Naturally, as the land continued to rise, a period arrived when the communication we have alluded to between the two seas became more and more shallow, and at last closed altogether. The impress left on the surface of the land by the changes which hence followed, is both marked and curious. Above the level previously indicated we have a stiff boulder clay – the former sea-bottom of a pent-up current similar probably to our own Pentland Firth. Below this level we find the natural products of a comparatively quiet sea – gravel, sand, and soft clay. When the sea had become shallow, but with the current still setting from the west through the Falkirk valley, the gravel might possibly preponderate; and this may have some connection with the great deposit of gravel known as the Redding Ridge, which extends from Laurieston on towards Linlithgow bridge, the causes leading to the formation of which, in this and other localities, are still matters of conjecture with scientific men. However, this deposit may have been formed, when once brought into existence, and raised above the sea level, it must have given rise to a somewhat extensive loch, or series of lochs, on its south side. One of the passages by which the water has escaped is well seen a short distance to the east of Polmont station; and through this gap the Gilston burn, from the upper grounds to the south, now takes its course.

After the entire stoppage of the current through the southern valley, a quiet, sheltered sea must have existed, into which the various streams – but especially the Carron, whose embouchure would then be above Denny – began and continued to pour down the debris of the high grounds which they drained. This process, it is clear, had lasted until a considerable part of the valley left by the former current had been filled up with such fine sand, gravel, and clay, as it at present contains; but still not in its present form, for as the land continued to rise, another agent came into operation which has had much to do in giving character and variety to the picturesque portion of country lying between Falkirk and Denny. As the potter with his handful of clay, and the turner with his rude piece of wood, bring out beautiful forms by a few apparently simple touches; so here nature, working with the sharp cutting edge of the Carron, has shapen this uniform sandy deposit into the lovely valleys lying along the course of the river, and has given us the exquisite rural scenery of Dorrator, Larbert, and Dunipace.

But what of the ancient sea-beaches which are thought to be so well seen on the course of the Carron? At Lock No. 2, on the Forth and Clyde Canal, and at Carron, we have well marked the 20 feet beach, upon the top of which stands Mungal Mill, as do also parts of Glasgow and Dundee. The same beach is finely seen at the foot of the Red Brae; and from this situation, looking towards Dorrator, may be had an excellent view of the 20, 40, and 53 feet beaches rising in succession above each other, and thus constituting the fine terraces which have long given a character to this part of the Falkirk neighbourhood.

The erosions of the Carron are best observed from the road between Larbert and Dunipace, in the direction of Carmuirs. In fact, the mounds at Dunipace are but evidences of the same eroding action, being composed of stratified sand, part of the original uniform deposit. The river, as we see, formed a passage for itself at Larbert church, and then appears to have been reflected southwards, cutting out at the Red Brae what is said to have been the site of the Roman port of ancient Camelon. Along a higher portion of the deposit, and between the two valleys, runs the Stirlingshire Midland Railway, and an inspection of the ground shows that had the eroding action continued much longer, this ridge would have disappeared, and the two valleys would have been laid into one.

A short time ago, an ancient river channel buried under drift, extending from Kilsyth to Grangemouth, was discovered through means of borings for minerals. Journals of these operations were collected for the purpose of ascertaining the depth and character of the surface deposits of the country; and it was while examining the same that the incidental discovery was made of a deep pre-glacial, or perhaps inter-glacial, trough or hollow, extending from the Clyde above Bowling, by Kilsyth, to the Forth, near Grangemouth. It is clear that this hollow was due purely to denudation, as the strata which it intersects was found to be intact and unbroken beneath – consequently cut out of the solid rock. It was at first supposed that the denuding agent might be the sea; but be it observed, that however effectually a sea-current might deepen and widen this trough where it was narrowest, or shallowest – that is, in the tract between Kilsyth and Castlecary – it could not have hollowed it out at either end, as these parts must have been, in that case, sunk about 410 feet below sea level, and, consequently far beneath the eroding action of the current. Moreover, it is quite contrary to the ordinary action of sea currents that they should cut out in the comparatively flat bottom they flow over a long channel, the sides of which are everywhere steep, and in some places perpendicular, and even overhanging. For these and other reasons, it may be concluded that this hollow had been cut out by running water in the form of rivers, when the land stood higher than now. These rivers, starting from the present watershed of the district near Kilsyth, would run, the one westward, flowing along the valley of the Kelvin, into the Clyde, near Bowling; and the other eastward, along the present course of the Bonny Water, till it entered the Firth of Forth. The geological state of this ancient river channel is shown by the deepest bore at Grangemouth to be either just before, or shortly after the beginning of the glacial epoch, which conclusion is confirmed by the deepest bore in the western portion of the hollow. The chief geological value of this discovery consists in the evidence it affords, that at the time when water flowed down this ancient river channel into the sea, the land must have stood nearly 300 feet higher than at present. The surface of the land at Grangemouth is only 12 feet above the level of the sea; and as the bottom of this old river channel is 273 feet below the surface, it is evident that the land must have stood about 260 feet higher than now. It is satisfactory thus to find on land a confirmation of what has long been inferred from the mammalian and other remains found in the German Ocean, the English Channel, and other parts, that at a very recent period our island must have stood several hundred feet higher than at present, and formed part of the great eastern continent, which then included in its area the present isolated lands of Great Britain and Ireland.

Several districts of the county offer happy hunting ground for the geologist. Here and there many rare specimens of minerals may be found. We make a running survey, starting from the west. The mountains of Buchanan parish, like the rest of the Grampian range, belong to the primary formation. Roof-slate and lime frequently occur, but schistus is the chief mineral. – The base of the Killearn district is the old red sandstone. In the rising ground a variety of strata is exposed to view by the action of the mountain streams, such as clay, lime, and freestone. In the trap formation, near the south end of the parish, there is a singular chasm called the Wanzie. A transverse section of a hill, running east and west, seems to have slipped off; probably from the partial decay of the subjacent sandstone leaving it without support. The chasm is 346 feet in length, 10 feet in greatest width, and 30 feet in present depth. Attempts have frequently been made to find coal in this neighbourhood. That they have always been unsuccessful is what should be expected from the mineralogy of the district. Coal is never found where the old red sandstone forms the base. – Fintry affords a great variety of minerals. Coal, in small seams, is found in many places; and granite occurs in detached fragments. There are also whinstone, freestone, redstone, jasper, and fine specimens of zeolite. The rocks, which belong to the trap formation, are numerous, and lie in a position very similar to those of Stirling castle, Craigforth, and others in that locality. – The rocks of the Lennox hills are composed chiefly of trap, or whinstone. In the gently sloping lands, between the hills and the carse, strata of red and white sandstone are everywhere found. – The Ochils consist of trap. The beds are of various thickness, nearly vertical, having their dip to the south. The veins, with a few exceptions, run in a northerly direction. The amygdaloid rock is abundant, with agates and calcareous spar. Along the face of the hills, and partially to the westward, is a thick bed of conglomerate rock, or breccia, having a dark brown coloured arenaceous base, in which are embedded fragments of trap rock, chiefly angular. The greenstone, of which the Abbey Craig is for the most part composed, is of felspar and horn-blende, and when broken presents a rough crystallized appearance. – In Baldernock the minerals are coal, lime, ironstone of various kinds, fireclay, pyrites, and alum-ore. Coal and lime have been wrought here for more than 200 years. - The Campsie Fells consist principally of large tubular masses of trap. The minerals of the district also include those of the coal formation – coal, with the usual alternating rocks of freestone, limestone, argillaceous ironstone, aluminous clay, slate, &c., with beds of fossil shells. In the Kirkton glen there is a very fine section of a dyke of compact felspar, about 20 feet in height and 5 feet in breadth, elevating the strata of limestone, slate-clay, and ironstone which bear upon the dyke, and dip on either side of it at a considerable angle. – The greater part of Kilsyth parish is a coal-field, but the mineral is not of so much value as might be anticipated from the broken nature of the strata by dykes and hitches. The principal dyke is known to run into the river Forth, near Airth, and commences here at Tomphin. It varies both in material and thickness. At some places it is 30 feet thick, at others double that number of yards. In one part it consists of the hardest basalts; at another of the softest blaes; and again, it appears as a mass of freestone debris, or whinstone interspersed with balls. There are also large ironstone fields at the two baronies and Banton; while limestone is found at Riskend. – At Denny the rocks are whinstone and freestone; but both coal and ironstone mines are wrought in the parish. – The whole of the easter part of Larbert parish is well stored with coal. Five seams have been discovered, and more or less wrought. The lowest of these dips out on the wester part of the Kinnaird and Carronhall estates. The dip is usually to the north-east, and the coalfield is intersected by several dykes. A fine freestone was formerly obtained at Carronhall, situated considerably above the highest seam of coal. This quarry, however, has been filled up. – All the rocks in the parish of Airth are of the coal formation, and form a part of the great coalfield of Scotland. The line of dip varies. Next to the Forth it is southerly, but in the south part of the parish it is towards the north-east. The sandstones are of various shades of yellow and grey, while some approach to white. – Coal is found in the higher districts of Falkirk in great abundance. Also ironstone, limestone, and sandstone. – Polmont contains coal, freestone, ironstone and fireclay. Freestone, however, is the only rock which extends nearly throughout the whole parish. The dip of the strata is generally to the north-east, except when their position is altered by a dyke which traverses one part of the parish, as exemplified in Brighton’s quarry, where the strata, in consequence, dip to the north-west.

We need hardly say that our pen has done little more than skirt the fringe of this interesting subject. But in geological matters we occupy the position of the unlearned.