Transactions Geological Society of Glasgow
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Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on June 14, 2015 TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF GLASGOW. I.—NOTES on a TILL or BOULDER CLAY with BROKEN SHELLS, in the Lower Valley of the River Endrick, near Loch Lomond, and its Relation to certain other Glacial Deposits. By ROBERT L. JACK, F.G.S., of tlie Geological Survey of Scotland* [Read loth Jan., 1874] THE eastern angle of Loch Lomond is at present being rapidly silted tip with the detritus carried into it by the River Endrick. Many acres of land have been gained from the Loch during the lifetime of some of the present residents on its shores. From the eastern extremity of the Loch there stretches, for some miles inland, a broad alluvial flat, through which the lower part of the Endrick pursues its crooked course. It is impossible to doubt that when the great icy graving tool was withdrawn, an arm of the newly made lake covered the site of this extensive alluvial flat. During the intensest cold of the glacial period, the valley of the Falloch (the site of Loch Lomond) was filled by an ice- sheet of immense thickness, which scored the side of Ben Lomond with strise (parallel to the direction of the valley) to the height of 2,250 feet above, while it grated over a bottom no less than 600 feet below the present level of the sea. At this period the Menteith Hills, which rise to 1,400 feet, and whose tops present a perfect example of a moutonne outline on a large scale,f * Of the facts mentioned in the following paper, those which were dis covered in carrying on the Geological Survey are communicated, with.the permission of the Director-General, in anticipation of the official Maps and Memoirs. f This appearance is so marked from the southern side of the Carse of Stirling, that the range is known to the inhabitants of Kippen and Bnchlyvie as "The Scolloped Hills." Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on June 14, 2015 6 TRANSACTIONS OF THE GEOLOGICAL SOC. OF GLASGOW. were overridden, and the Campsie and Kilpatrick Hills, though rising to 1,894 feet, were buried beneath the sheet of ice which flowed south-eastward out of the Highland glens. By contouring the bottom of the lake (from the data afforded by the soundings on the Admiralty Chart), I have found that there are no less than twelve separate basins between Ardlui and Balloch. These hollows are, without exception, elongated in the direction of the valley, and follow pretty nearly the sinuous line laid down on the Ordnance map as the boundary of the counties of Dumbarton and Stirling—a line which probably corresponds pretty closely to the course of the Falloch before Loch Lomond existed. The hollows are divided one from another, not by steep scarps, but by undulations so gentle that when the contour of the bottom is drawn to a true scale, they are hardly appreciable by the naked eye.* The shallows and deeps, when the geology of the neigh bourhood comes to be examined in detail, will probably be found to correspond respectively with the outcrops of harder and softer bands of the upturned Silurian strata, which strike across the lake from south-west to north-east, and they may also be supposed to depend in some degree on the varying width of the preglacial valley. It is not, however, my intention at present to recapitulate the evidence on which Loch Lomond is claimed as an example of a glacier-formed lake, but merely to make such a sketch of the physical geography of the district as will render intelligible the peculiarities of the deposits to be described. It has already been observed, that when the ice-sheet was at its. thickest it overflowed the Menteith, Campsie, and Kil patrick Hills, and this is attested by the glacial striae and the " carry " of the drift. But, as Mr. Dugald Bell has pointed out,f there were periods when the attenuated ice-sheet was divided by the minor hill-ranges that opposed its course, and was forced to round them and to stream through the valleys intersecting them from west to east. This view, I may say, is amply supported by the strise mapped by myself in carrying on the Geological Survey^ the high grounds being crossed by striations from * See Mr. J. Geikie's Great Ice Age. Appendix D. + " Notes on Cross Striae at Possil, near Glasgow."— Trans. Geol. JSoc, Glasgow, Vol. IV., pp. 223 aud 304. t The striae referred to will be found in the forthcoming Geological Survey Sheets, Nos. 30 and 31. Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on June 14, 2015 JACK—ON A SHELLY TILL OR BOULDER CLAY. 7 N.W. to S.E., while the lower Clyde Yalley, the Blane-Glazert Yalley, the Endrick-Carron Yalley, and the Forth Yalley are swept by east and west striations. When the ice had degenerated from a continuous "nappe" bearing down (in this district) from the N.N.W. to a series of separate glaciers streaming from west to east over the low passes of the Campsie Hills, the erosion of a new set of lakes would be begun nearly at right angles to the older set, and this erosion would be much facilitated by the circumstance that the courses of the protruding tongues of the shrinking ice-sheet would lie mainly among Sandstones of Lower Old Red and Lower Carboniferous age, much softer than the Silurian rocks enclosing Loch Lomond in its upper part. Theoretically, we should look for lakes in localities where the ice-streams or glaciers would be subjected to strangulation from the narrowing of their respective beds, and accordingly we find broad alluvial flats in several such situations, which it requires no great stretch of imagination to regard as silted-up lake basins, viz. :—in the Endrick-Blane Yalley, the one above referred to as flanking the lower "reaches of the Endrick, and which is merely a prolongation of Loch Lomond ; in the Blane Yalley proper, the one extending from the confluence of the river with the Endrick to Duntreath Castle; and in the Endrick Yalley, the one stretching from Newtown of Fintry to the Gonachan. In the case of the Dun treath Castle basin, the cause of " strangulation" is obviously to be found in the hard trap escarpments and magnificent volcanic "necks" overlooking Strathblane. The sudden contraction of the Endrick Yalley at its issue from between the fine trap escarpments at Fintry could not fail to increase the intensity of the erosion on the site of the Gonachan basin. It is not impos sible that the rock-basin of Loch Lomond may be continuous from the mouth of the Endrick up the Blane Yalley to Duntreath Castle, as the rock is nowhere seen in the bottom of the valley, and the lower part of the valley is flanked by deep drift deposits, which may conceal a broad rock-basin. It is, however, probable that a gentle rise of the Sandstone bottom about Catter Burn- foot, divides this corner of the rock-basin (which, had the country beyond offered a level bed for the passage of the ice, would naturally have terminated the loch), from the depression higher up the valley, due to the strangulation of the glacier. The elevation (20 feet) of Loch Lomond above the sea is so Downloaded from http://trngl.lyellcollection.org/ at Carleton University Library on June 14, 2015 8 TRANSACTIONS OP THE GEOLOGICAL SOC. OP GLASGOW. trifling that there is no difficulty in classing it with the sea lochs which indent the western coast of Scotland. All these sea lochs are shallow at their outlets, and would become fresh-water lakes, like Loch Lomond, on a very slight elevation of the land. In fact, it may be regarded as in a manner an accident that Loch Lomond is not now an arm of the sea. A depression of 20 feet, or the removal of the superficial deposits traversed by its short out let, the Leven, would restore it to its former condition. In the course of my work on the Geological Survey to the south of the eastern angle of the Loch, and its silted-up continu ation above referred to, it fell to my lot to map out a deposit whose relation to the already known members of the glacial series seemed to deserve particular attention. The deposit is a true typical Till, in every respect analogous to the Old Boulder Clay or Till of the Lowlands of Scotland. In a matrix of stiff unstratified clay, brown in colour, like the subja cent Old Bed Sandstone rock, are scattered stones of varying sizes up to several feet in diameter. These are blunted, smoothed, and marked with striations in all directions, but most frequently in the direction of their longer axes. The Till under goes local variations, but on the whole may be described as above. It presents, however, one remarkable peculiarity to dis tinguish it from the common Till of the country. It contains worn and broken fragments of marine shells. I shall now describe briefly the various exposures of the Till which have come under my observation. I am indebted to Mr. Robert Etheridge, Jun., F.G.S., Acting Palaeontologist to the Geological Survey of Scotland, for naming some of the shells mentioned in this paper, and for many valuable notes. Messrs. J. Bennie and A. Macconochie also obliged me by washing a quantity of the Till for minute organisms.