38 VI.—The Intercrossing of Boulders in the Applecross Mountains. by J

Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015 38 EDINBURGH GEOLOGICAL SOCIETY. VI.—The Intercrossing of Boulders in the Applecross Mountains. By J. HORNE, H.M. Geological Survey of Scotland. (Communicated by permission of the Director-General of the Geological Survey.) (Read 15th February 1894.) CONTENTS. 1. INTRODUCTION. 2. PHYSICAL FEATURES OP THE APPLECROSS MOUNTAINS. 3. DIRECTION OP STRIAE ON THE LOFTY PLATEAU DURING THE EXTREME GLACIATION. 4. TRANSPORT OF MATERIALS DURING THE GREATEST GLACIATION. 5. SOURCES FROM WHICH THE FOREIGN MATERIALS WERE DERIVED. 6. PROBABLE CAUSE OF THE DEFLECTION OF THE ICE-SHEET IN APPLE- CROSS. 7. ESTIMATE OF THICKNESS OF HIGHLAND ICE-SHEET. 8. VALLEY GLACIERS. 9. SUMMARY. 1. A study of the glacial phenomena of the south-western portion of the Applecross peninsula furnishes important evidence regarding the massiveness of the ice-sheet during the maximum glaciation, and the transport of boulders in opposite directions during successive stages in the history of that period. In the area north-west of Loch Kishorn there is clear proof of one set of boulders being carried up the valleys facing that loch and across the water-shed towards the Inner Sound of Eaasay, while another set of local boulders has been borne down the same valleys by the later glaciers. Still more striking is the fact that foreign boulders have been first carried up and then carried down one and the same valley. Such phenomena bear a close resemblance to some of those described by MM. Falsan and Chantre in their great work on the ancient glaciers of the basin of the Ehone, where they attribute them to a change in the direction of the ice flow at different stages of the. Ice Age. 2. The group of pyramidal mountains forming the southern portion of the Applecross peninsula rises abruptly from the shores of Loch Kishorn, and from the alluvial flats at the month of the Kishorn river, to heights varying from 2000 to nearly 3000 feet. Composed of gently inclined beds of Torridon Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015 INTERCROSSING OF BOULDERS IN THE APPLECROSS MOUNTAINS. 39 sandstone, this range of mountains furnishes remarkable examples of the influence of parallel beds of strata in the for mation of scenery. The gloomy corries and the deep narrow valleys are encircled by long lines of escarpment that in some instances tower for 1500 feet above the floor of the valley. An observer, stationed on the watershed between Loch Carron and Loch Kishorn, looking across to that noble group of moun tains, is at first reluctant to believe that during the maximum glaciation the ice must have crossed Loch Kishorn and the river at the head of that sea-loch, ascended the valleys, and ulti mately passed over the watershed in a NW. direction towards the Inner Sound of Eaasay, And yet, from a consideration of the stride and the transport of stones, such a conclusion seems inevitable. The whole of the south-western portion of the peninsula was mapped by me in the course of my official work eastwards to the following boundary—viz., a line drawn from the head of Loch Kishorn north-west by Loch Goire nam Faradh to Allt Coire Attadale. The tract to the east of this limit, including Ben Bhan (2936 feet)—the most elevated part of the whole peninsula—was surveyed by my colleague, Mr Hinxman. The evidence obtained by him on that mountain is remarkable. Just underneath the 2750 feet contour line he found striae pointing W. 20° K and also W. 13° N., with an occasional block of Eastern Gneiss between that level and 2500 feet. Below the latter elevation erratics of Eastern Gneiss are fairly plentiful. 3. In the area examined by myself, on the col between Coire Attadale and Coire nam Faradh at a height of 2000 feet, the direction of the ice-markings is N. 21° W., and on Carn Dearg, between 1750 and 2000 feet contour lines, the trend varies from K 41° W. to W. 41° K Again, on the northern spur of Sgorr na Caorach at the same elevations, the markings point W. 23° N. and W. 34° N. Proceeding westwards to the lofty plateau crossed by the road leading from Kishorn to Applecross, various striated surfaces are observable about a height of 2000 feet. The direction, varies from W. 20°—43° N.', but in some instances the local glaciers have partly effaced these earlier markings. For on the plateau above the zig-zag windings of the Applecross road there is a small corrie (Coire nan Clach) filled with a small group of local moraines and thickly strewn with boulders. The stream draining the western slope of the highest peak of Sgorr na Caorach (2539 feet) flows into this corrie. Now, close by the road excellent examples of cross-hatching are visible; the older set pointing W. 30° N., and the later or valley glacier Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015 40 EDINBURGH GEOLOGICAL SOCIETY. markings S. 28° W. Similar evidence regarding the north westerly flow of the ice is found on Crag Gorm at a height of 1750 feet, and when we pass westwards to Cam Chailean (1750 feet) the sandstone escarpments of that eminence are finely striated in a direction varying from W. 26° to 28° N. On referring to the Ordnance Survey map of that region, it will be seen that the watershed from Ben Bhan, by Cam Dearg, Sgorr na Caorach, Meall Gorm to Cam Chailean, forms a sinuous line upwards of five miles in length, embracing the most elevated ground in the peninsula. And yet this lofty plateau has been striated in one general direction, varying from WW. to NW., and in some rare instances to NNW.; the striae being found at all levels from 1750 feet to 2750 feet. Confirmatory evidence can be gleaned from the glaciated sandstone surfaces that slope westwards to Applecross, Camasterach, and Toscaig. Even by the shores of Loch Toscaig the trend is W. 15° K, and near Calduie, close by Camasterach, it varies from W. 40° N. to N. 31° W. My colleague, Mr Peach, who mapped the greater part of the peninsula lying to the north of the midland valley, obtained similar evidence regarding the north-westerly flow of the ice. It is clear, therefore, that these north-westerly striae belong to one general system, when the peninsula was glaciated in one determinate direction more or less independently of its physical features. Fortunately, by means of the transport of erratics, we are able to determine with certainty that the move ment during this period of maximum glaciation must have been from the south-east towards the north-west. 4. Eeference has already been made to the occurrence of blocks of Eastern schist and gneiss on Ben Bhan below the 2500 feet level. On the highest peak of Sgorr na Caorach, between the 2250 and 2500 feet contour lines, there are numerous fragments of deformed Lewisian gneiss, mylonised gneiss, thrust Torridon sandstone, slightly schistose and traversed with quartz-strings, with crystalline schists of the type of the Moine schist. One boulder—the largest noted, measuring about 12 cubic feet— is composed of crushed grey gneiss of the type occuring m situ north of Glenmore and north-east of Loch Kishorn. These foreign materials are very plentiful on the top of Cam Dearg, above the 2000 feet level, on Crag Gorm, and on the water shed southwards to Cam Chailean. They are specially abundant on the cols at the head of Coire nam Faradh and Alp a' Chumhaing—the two main valleys draining into Loch Kishorn. They are strewn over the rocky slopes northwards to the village of Applecross, and they occur in the fine boulder clay sections in Allt Mor that joins the Applecross river at Hartfield. They have likewise been traced over the north-western part of the peninsula by Mr Peach. Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015 INTERCROSSING OF BOULDERS IN THE APPLECROSS MOUNTAINS. 41 5. The detailed inapping of the mountainous region of that part of the west of Eoss-shire enables us to fix the sources from which these materials were derived. The blocks of deformed grey gneiss, mylonised Lewisian gneiss, and green and grey schistose Torridon sandstone, belonging to the lower division of that formation have been obtained from regions lying to the east of the great terrestrial movements in the north-west High lands. My colleague, Mr Hinxman has traced the outcrop of one of these maximum-thrust planes from the shores of Loch Kishorn northwards, and then eastwards to Loch Carron. By means of this displacement a belt of deformed Lewisian gneiss and Torridon sandstone has be&n made to overlie the piled-up Cambrian limestone, quartzites, &c. Now the materials, con sisting of deformed gneiss and schistose Torridon sandstone, that were transported by the ice across the south-western portion of the Applecross peninsula must have been derived from the dis placed masses east of this thrust-plane between Kishorn and Loch Carron. The crystalline Moine schist occurs in situ to the east of the deformed Lewisian rocks, at the village of Loch Carron; while the main outcrop forms the mountains to the east of Auchnashellach and Strathcarron stations. Nowhere within the region of Applecross are rocks of these lithological characters to be found in situ. It is thus apparent that the evidence derived from the transport of erratics is in accordance with that of the striated surfaces and roches m>ontonnees.

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