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Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on July 1, 2015 TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF GLASGOW. No. XYIII.—THE AGE OF LOCH LONG, AND ITS RELATION TO THE VALLEY SYSTEM OP SOUTHERN SCOTLAND. By Professor J. W. GREGORY, F.R.S., D.Sc. [Read 14th January. 1915; issued separately 25th February, 1915.] I, THE ORIGIN OF THE VALLEY SYSTEM OF THE LOCH LONG DISTRICT. LOCH LONG has been often called a fiord, and is the most typical fiord in south-western Scotland. It is. easy of access, being traversed by the West Highland Railway, and it affords an excellent test case as to the extent to which the valleys of this part of Scotland have been deepened and their hanging valleys have been caused by glacial excavation. The mouth of the hanging valley of Coilessan Glen is a little over 500 feet above Loch Long, which is there 30 fathoms deep. This difference of level is attributed by one theory to the glacial deepening of the main valley to the extent of about 700 feet. There is, however, an alternative explanation, according to which Loch Long lies in a pre-glacial valley that need not have been much deepened by glacial excavation. This explanation was briefly stated in my work on Fiords (1913, p. 437), but as no illustrations or details) of Loch Long were there given it may be as well to state the evidence. The origin of the valley system to which Loch Long belongs was first explained in a most suggestive paper by Mr. Cadell in VOL. xv., PT. HI. x Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on July 1, 2015 298 TRANS.—GEOLOGICAL SOCIETY OF GLASGOW. [Vol. XV. 1886.1 To understand the origin of these valleys it is necessary to consider the geographical condition of western Scotland in early Kainozoic times. During the Eocene or Oligocene Periods —the evidence, though uncertain, appears to indicate the latter date—the Western Isles of Scotland were a field of great volcanic activity. After the eruptions the volcanic area probably sank; but before these subsidences the level of western Scotland must have been higher than it is now, and from this high land many rivers flowed south-eastward. As Mr. Cadell remarked, the valleys on the site of Loch Goil and its continuation, the Gare- loch, of Glen Croe and the Tarbet gap, and of Loch Eck with the Holy Loch, were probably excavated by rivers flowing from the north-west into the Clyde Valley. The First Valley System.—Between Crianlarich and Dunoon there are nine valleys which may be included in this original system (PL XXXL). These valleys are in order the Dubh Eas- Glengyle, Glen Sloy-Loch Arklet, Glen Croe and the Tarbet gap, Coilessan Glen continued by Glen Douglas,2 Glen Luss, Glen Fruin, the valley of Lochgoil and the Gareloch, Glen Finnart and the lower reach of Loch Long, and, finally, Loch Eck and the Holy Loch. Dubh Eas, which rises in the high ground near Ben Lui, would have been continued across Glen Falloch into AUt Innse ; thence, up the tributary of the Innse to the south-west of Parland Hill, it would have crossed the windgap into Glen Gyle, and thus passed to Loch Katrine. This stream would have received as tributaries on its south-western bank the AUt Arnan and the Dubh Uisge. 'The valleys of the Dubh Eas, Allt Arnan, and the Dubh Uisge converge toward the mouth of the Allt Innse, on the eastern side of Glen Falloch. The lower Allt Innse follows the direction of Dubh Eas to the pass leading to Glen Gyle. The river in Glen Sloy, the Inveruglas, was probably joined by the two streams from Allt Coiregrogain that now discharge through Glen Loin to Loch Long; but the 1 Further details are given in Mr. Cadell's "The Story of the Forth" (1913, pp. 56-80); except for some secondary points, his conclusions regarding the rivers from Glen Sloy southward are the same as those adopted in this paper. 2 The former continuation of these two glens was suggested by Cadell, who says that they are "exactly in line" (Cadell, 1886, p, 346). Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on July 1, 2015 Sketch Map of the Loch Long District. The probable course of the valleys which date from the primitive Lower Kaino/oic river system. Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on July 1, 2015 Part iii.] GREGORY—THE AGE OP LOCH LONG. 299 Inveruglas probably originally crossed the site of Loch Lomond, and passed through the hanging valley at Inversnaid to Loch Arklet, and thus to Loch Katrine. Glen Croe, as suggested by Cadell, once continued through the Tarbet gap, towards which the U-shaped section of the glen above Larachpark points directly. The trend of the Tarbet gap to the north of east and of the south-eastern face of Cruach Tarbet, above the contour of- 600 feet, to the north-east may both be due to the course of the Glencroe-Tarbet river when it joined the Inveruglas and discharged through the Loch Arklet gap. The stream from Coilessan Glen doubtless at the same time continued down Glen Douglas, and, turning to the south, was joined by the Luss and Fruin Waters, and flowed into the Clyde. The next of this series of streams ran through Hell's Glen to the Gareloch. It has now been subdivided, for Hell's Glen has been left as a hanging valley by the deepening of Allt Glinne Mhoir, and Loch Goil has been separated from the Gareloch by the extension of the outer reach of Loch Long. That part of Loch Long was probably initiated as the lower end of Glen Finnart. The most southern of this series of valleys is that of the Holy Loch and Loch Eck, which is continued to Loch Fyne over the deeply cut windgap at Strachur. Level of the Old Valleys.—The courses suggested for these rivers were only possible so long as the levels were higher on the western than on the eastern side of the Lomond-Falloch Valley. The hanging valley of Dubh Eas now ends at the height of between 550 and 600 feet, whereas the mouth of the hanging valley on the opposite side is at about 700 feet, and the pass at its head, the Glengyle windgap, is at the height of a little below 1500 feet. The peaks around the head of Glen Eas, however, rise to heights of from 2400 to 3700 feet, and that glen has been cut in a plateau which was not less than 2400 feet high; and that height was sufficient to have given the original Dubh Eas an outlet across the Glengyle windgap. As soon as the Falloch Valley opposite Dubh Eas was lowered beneath the 1500-feet contour the stream must have found another outlet, and this was probably at first north-eastward over Crianlarich Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on July 1, 2015 300 TRANS.—GEOLOGICAL SOCIETY OP GLASGOW. [Vol. XV. to the valley of the Dochart.3 Similarly, the Glen Croe and Glen Loin rivers can only have discharged through Loch Arklet when their beds were above the level of 470 feet. The whole of the Loch Long district was a high plateau, the level of which was over 2000 feet from Glen Falloch to Glen Fruin and the Holy Loch; and to the south of the two last glens it was at the height of over 1700 feet—the height still reached by the Renfrewshire Hills (1711 feet) and by the hills (1713 feet) west of Dunoon. The Oligocene Valleys were cut into this high plateau; the beds of the valleys were therefore originally not less than 1700 feet above present sea-level. The denudation of the subsequent valleys of Loch Long and Loch Lomond was, however, probably accomplished in Pliocene times, when the land stood at a much higher level. This uplift is generally accepted4 in order to explain the platform at the height of from 800 to 1000 feet, for which there is abundant evidence throughout Scotland. If the country then stood so much above present sea-level, the formation of the inner basin of Loch Long, which sinks 180 feet below O.D., and even the northern basin of Loch Lomond, which is over 600 feet deep, might have been due to river erosion. II. EXTENSION OP THE VALLEYS ACROSS LOCH FYNE AND TO THE TWEED. Mr. Cadell traced this river system north-westward as far as the south-eastern side of Loch Fyne, where he placed the original watershed; hut the principle on which his conclusions were founded admits of the further extension of these rivers in both directions. The south-eastern side of the Loch Fyne Valley is notched by windgaps between Strachur and Loch Eck at the level of only 186 feet, at the head of Hell's Glen at 727 feet, and between Kinglas and Glen Croe at 860 feet. The depths of these three notches beneath the adjacent peaks are respectively 800, 1300, and 2100 feet. 81 am glad to find that Messrs. A. Scott and B. K. N. Wyllie have independently come to the conclusion that Dubh Uisge once discharged to Glen Dochart. 4 This is the Intermediate Plateau of Peach and Horne (1910, in Murray and PuUar, vol. i., p. 458). Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on July 1, 2015 Part iii.] GREGORY—THE AGE OP LOCH LONG.