By ROBERT BOYLE, B.SC, A.G.T.C., President of Glasgow University Geological Society
Total Page：16
File Type：pdf, Size：1020Kb
344 TRANSACTIONS GEOLOGICAL SOCIETY OF GLASGOW. No. XXIX.—THE ECONOMIC AND PETROGRAPHIC GEOLOGY OF THE NEW RED SANDSTONES OF THE SOUTH AND WEST OF SCOTLAND. By ROBERT BOYLE, B.SC, A.G.T.C., President of Glasgow University Geological Society. [Read 27th May, 1909.] I. INTRODUCTION. THERE are in the South and West of Scotland eight separate areas of some considerable extent occupied by red sandstones of post-Carboniferous date. Six of these areas have been economically developed. That they are younger than the Coal Measure sandstones is certain, but they have been variously regarded as of Permian or Triassic age. Whatever their age may be, their characteristics are very similar, and they will here be referred to as " New Red Sandstones." The various districts may be separately distinguished as—(1) Arran, (2) Mauchline Basin, (3) Thornhill Basin, (4) Dumfries Basin, (5) Annan Basin, (6) Lochmaben and Corncockle Moor, (7) Moffat, (8) Ballantrae. They consist of bright brick-red, red, or orange-red sandstones, with abundant false bedding, associated with more or less volcanic material at their base. They will be referred to in the order given. There seems good reason to believe that they were originally all connected, their subsequent isolation being due to subserial denudation. Goodchild (Trans. Geol. Soc. of Glasgow, vol. xi., part i., page 96—" Desert Conditions in Britain ") considers that the new red rocks, and even the younger Jurassic rocks, formerly covered nearly the whole of Britain; and Sir A. Geikie (" Scenery of Scotland ") mentions the discovery of fossils belonging to the Avicula contorta zone of the Rhaetic group as indicating that the Arran sandstones and marls belong to the Triassic period. He further says that these strata no doubt at one time extended over a much wider area of the Midland valley. A discovery by Messrs. Peach and Gunn of fragments of material, sealed in a volcanic neck, belonging to BOYLE—ECONOMIC GEOLOGY OF THE NEW RED SANDSTONES. 345 the Rhsetic and Lower Lias shales, and of hard, white lime stone with Cretaceous foraminifera, shows that the Trias, Lower Lias, and Chalk once stretched over the region of the Clyde. The objections to considering the New Red Sandstone series of Mauchline to be of Triassic age are—(a) They form one continuous series with the Coal Measures; (b) they are asso ciated with considerable contemporaneous volcanic action, which is unknown elsewhere in the British or German Trias. The same holds good to a large extent in the north part of the Thornhill area, and in its prolongation along the Carron water valley. As will afterwards be shown, there is a general similarity amongst all these rocks, except in the Annan area, which would indicate that they were of the same age, or at least formed in periods which were climatically the same, and without any break. On the whole the age seems more likely to be Permian. It is mainly the areas of economic importance which will be dealt with. There are, in addition, scattered patches of red breccias and sandstones in the valleys of Nithsdale and Annandale, in a little basin among the Leadhills and in Loch Ryan, Wigtownshire, which are either of Permian or Triassic age. These seem to point to the original large extent of area covered by these deposits and the great amount of denudation which has taken place. II. GENERAL DESCRIPTIONS OF AREAS. 1. Arran.—In 1894 it was first observed that the red rocks of the south of Arran were unconformable to the underlying Carboniferous beds. The lower sandstones resemble those at Mauchline, and are extremely false bedded. The thickness is about 1000 feet, being about one-half of those beds supposed to belong to the Lower Trias. The lowest subdivision of this series is well represented between Corrie and Brodick Castle. The sandstone contains ridges of hardened joints, crushes, and faults, and occasionally many branching veins of hardened material, which cause the rock to weather with a honeycomb structure. The ridges are supposed to be due to igneous action, perhaps that of dykes beneath the present surface. The colour is a characteristic bright brick-red, and there is much 346 TRANSACTIONS GEOLOGICAL SOCIETY OF GLASGOW. false bedding. The outlier at the Cock of Arran, in the north of the island, is similar to the Corrie rock. It occupies an area of about § of a square mile, and the formation rises to about 1000 feet above sea-level. There are beds of con glomerate overlying it, and bands of the same material at its base. Areas of the New Red Sandstone are scattered all over the island. Besides the whole eastern shore south of Corrie, it is found as far west as Gleann Dubh, where fine crags have been formed, and also well up Benlister Glen. On the west side of the island it is fouad along the coast south of Auchagallan, and it spreads over the vale of Shiskine on either side of the mouth of the Machrie Water. Near the latter there are many cavities of various shapes and sizes in the sand stone, known as " moss holes." The various districts are very fully described in the Arran Memoir of the Geological Survey. Although the possibilities of the island are very great, quarries have only been opened at Corrie and Brodick Wood, at the back of the school. " Whin " is being quarried for road metal right in the middle of the Corrie quarries, and the sandstone has included in it elongated pieces of very hard rock, accompanying what are known as " moss patches/' which may be as large as 18 inches to 2 feet. These portions are dark in colour, and are probably hardened, due to an excess of carbonates in the sand produced by segregation. The top decomposed rock, variously called " bare," " tirr," and " strip,'' is about 4 feet. The quality appreciably improves with the depth, but the small, black carbon spots, common to most red rocks, become more numerous. The greatest depth worked is about 70 or 80 feet. False bedding is very abundant, and an angle of 40 degrees is not uncommon. There are numerous thin, hard, ramifying bands in the rock, locally known as " glass bands," from the thickness of a thread up to \ inch or more. They appear to be of a calcareous nature, and are probably due to deposition from solutions. Igneous action may in some cases have had to do with their origin. They spoil the rock considerably for working. The quarries at Corrie have been closed for about fourteen months, but Corrie rock is the principal building stone in Arran. Fairly large blocks can be obtained. The grain is rather coarser than most BOYLE—ECONOMIC GEOLOGY OF THE NEW RED SANDSTONES. 347 of the New Red Sandstones, but the texture is very uniform. It is soft and easily worked, and hardens by exposure to the air. It is softer than Locharbriggs rock. Much of the stone is shipped, a good deal of it coming to Ayrshire and the Clyde district. Troon harbour is said to have been built of rock from the northern quarry. The School of Art and Simpson's warehouse in Glasgow have been built of Corrie stone. 2. Mauchline Basin.—The New Red Sandstones in Ayrshire occupy an area of about 18 square miles. The outline is pear-shaped, and extends from Mauchline westwards to near Tarbolton, and southwards as far as Trabboch Burn. They are encircled by a ring of volcanic rocks, which form a rim to the basin, varying from \ to \\ miles wide. This igneous rock consists of porphyrite, melaphyre, and tuff, and dips down underneath the sandstones. Volcanic necks which belong to the same period of activity occur scattered through Ayrshire and Nithsdale, in Dumfriesshire, as, for example, at Sorn Hill, Stevenston near Ochiltree, Whitehill near New Cumnock, and at Blackeddie, Crawick Bridge, and the district north of Sanquhar. Beneath these igneous rocks comes the Coal Measure series, consisting of red sandstones, shales, and seams of coal; and in all probability there is a large area of workable coal several hundred fathoms beneath the surface in the Mauchline area. It may be affected in many places by volcanic action and burning due to intrusions in the same and later periods. Some of the necks may, indeed, be covered over with the sandstone and thereby hidden, as the volcanic activity seems to have ceased largely before the formation of the sandstone had proceeded very far. From recent investi gations by Mr. D. Ferguson, F.R.G.S., M.Inst.M.E., described in a paper read before Glasgow University Geological Society, there can be little doubt that this is one of the great sources of coal in the near future. Its probable existence was pointed out as far back as 1879 by Sir A. Geikie before the Royal Commission on Coal. The upper seams beneath the centre of the basin would appear to be at a depth of approximately 400 fathoms, and underlying the Upper Red Sandstones of the Carboniferous, which again underlie the melaphyres and New VOL. XIII., PT. III. 1 B 348 TRANSACTIONS—GEOLOGICAL SOCIETY OF GLASGOW. » Red Sandstones of Mauchline district. The area now occupied by the latter has, near the close of the volcanic period, been occupied by a lake of considerable depth, in which current action was very marked. In this were deposited the beds of bright brick-red or orange-red sandstones, which are so characterised by abundant false bedding. In places the melaphyre can be seen intercalated with the sandstone, as on the right bank of the river Ayr, between Howford Bridge and Ballochmyle viaduct.