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38 EDINBURGH GEOLOGICAL SOCIETY.

VI.—The Intercrossing of Boulders in the Mountains. By J. HORNE, H.M. Geological Survey of . (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 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 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. Further evidence, though negative, might be adduced that the ice movement during the maximum glaciation must have been towards the north-west. At the mouth of the Applecross river there is a small outlier of Lias limestone and shales which has been minutely described by Professor Judd.1 Now, if the move­ ment had been from the north-west, blocks of these fossiliferous rocks would assuredly have been found to the south-east of the limits of the outlier. But not a single boulder of these materials has yet been found on the rocky slopes that stretch south- eastwards to the watershed. 6. Before proceeding to describe the distribution of materials by the later valley glaciers, we might pause for a moment to consider the probable cause of the deflection of the ice-sheet towards the north-west in Applecross, and the light which it throws on the question of the thickness of the mer de glace. From the western headlands of Applecross, an observer sees the smooth cones of the Eed Mountains and the serrated peaks of the Coolins in Skye, rising to 3000 feet in height; while far away to the north extends the plateau of the bedded basalts. It is a well-known fact that the present average rainfall in 1 See Q. J. Geol. Soc, Vol. xxxiv. p. 672. Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015

42 EDINBURGH GEOLOGICAL SOCIETY.

Skye is excessive, and it is now generally admitted that those regions where the present rainfall is abnormal may have been coincident with excessive precipitation of snow during the glacial period. If this reasoning be correct, then it is not improbable that during the greatest extension of the ice Skye may have been buried under a thick icy covering. The presence of this opposing force would undoubtedly exercise some influence on the ice radiating from the mainland. The latter in that portion of Koss-shire would follow the path of least resistance, which would be north-west towards the Inner Sound of Eaasay. 7. It seems to me that the foregoing evidence regarding the north-west movement in Applecross points to an enormous thickness of the icy covering on the mainland. When such an isolated group of lofty mountains was over-ridden by the ice, after crossing Loch Kishorn and the glen at the head of that sea-loch, it is obvious that no system of valley glaciers can account for such phenomena. My colleague, Mr Hinxman, informs me that the top of Ehuadh Stac (2919 feet), on the eastern side of Glen Kishorn, is finely striated; the markings trending W. 25° K He has further observed that the summit of Meall a Chinn Dearg (3095 feet) composed chiefly of Torridon grits and sandstones, is covered with transported blocks of Torri­ don sandstone and Cambrian quartzite. These are certainly remarkable facts. Indeed, when they are viewed in connection with the evidence in Sutherlandshire, where it would seem that the ice-shed during the extreme glaciation lay to the east of the present water-parting; and with the evidence in Applecross, where a group of mountains rising from the sea-level to nearly 3000 feet has been overtopped by the ice, the estimate of the thickness of the Highland ice-sheet given by Sir A. Geikie does not appear to be excessive. In the latest edition of his text­ book, he says: " Many mountains in the show marks of the ice-sheet at heights of 3000 feet and more. If to this depth we add that of the deep lakes and fjords which were filled with ice, we see that the sheet could not have been less than 5000 feet thick in the northern parts of Britain."1 8. Brief reference must now be made to the transport of materials by the local glaciers in the two main valleys draining the slopes of these mountains towards Loch Kish­ orn. One of these is traversed by the Allt na Fraoich (Burn of the Heather), which, rising in Coire nam Faradh on the col between Ben Bhan and Sgorr na Caorach, dis­ charges into the sea at the head of Loch Kishorn. The otheT 1 See "Text-Book of Geology." Sir A. Geikie. Third edit., 1893. p. 1027. Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015

INTERCROSSING OF BOULDERS IN THE APPLECROSS MOUNTAINS. 43 is watered by AUt a' Chumhaing, which drains the comes between Meall Gorm and Sgorr na Caorach. Below the limits reached by these two valley glaciers, the ice-markings are marvellously fresh, and in almost all cases they seem to indicate a movement towards Loch Kishorn. From the mouth of the latter stream, along the shore, west­ wards by Airidh-drishaig to within a mile of Uaigs, a distance of five miles, the striae are running more or less parallel with the coast. Whether these may indicate an " undertow " of the ice-sheet during the extreme glaciation, or merely a shrinkage of the ice moving parallel with the sea-loch, it is difficult to say. It is not improbable that the former view may be.the correct one for reasons which cannot here be given. The upper limits of the Coire nam Faradh glacier are well defined by the splendid lateral moraines. On the west side of the loch of that name (see 1-inch Ordinance map) there is a prominent group of crescent-shaped moraines at a height of several hundred feet above the loch, that form continuous ridges curving towards the centre of the valley. Each ridge, along its upper limit, "tails off" against a prominent escarpment of Torridon sandstone, and is strewn with boulders of that forma­ tion derived from the magnificent cliffs on the west side of the valley. At this point the local glacier must have been 500 feet thick. fi The eastern limit of this glacier is defined by a great lateral moraine traceable for half a mile from the sea-level at Loch Kishorn up the slope of Ben Bhan. That the glacier descending from Coire nam Faradh coalesced with that in the adjoining valley to the west as they approached the sea-level is highly probable from the behaviour of the moraines. Long lines of boulders mark the course of these glaciers towards Loch Kishorn. They are very conspicuous on the ground between Bussel and the mouth of Allt a Chumhaing. At the latter locality concentric lines of moraines and trains of boulders may be traced from the sea-level up the slope west of the stream to the rocky escarpments of Meall Gorm. Now, a careful examination of the various sections of moraine matter shows that the materials, with few exceptions, have been derived from the cliffs of Torridon sandstone in the respective valleys. The massive erratics forming the parallel lines of boulders have been carried for some distance down these valleys. There are, however, occasional ice-worn fragments of crushed Lewisian gneiss, mylonised gneiss, and schistose Torri­ don sandstone in the moraine matter; and I observed one or two larger blocks of crushed gneiss perched on the moraines on the slope west of Loch Coire nam Faradh. These materials were Downloaded from http://trned.lyellcollection.org/ at Purdue University Library on July 15, 2015

4A EDINBURGH GEOLOGICAL SOCIETY. distributed, as we have seen, during the great glaciation, and the debris must have fallen on to the surface of the later glaciers, and must have been carried down to lower levels, thus performing a double journey. It is1 a significant fact that above the limit of the valley glaciers on the southern slope of Sgorr na Caorach,.at levels ranging from 700 to 1000 feet, the debris of deformed Archaean gneiss, &c„ derived from the east of Kishorn, seems to be more abundant than in the moraine matter distributed by the later glaciers. 9, We might now summarise the evidence given in the fore­ going pages regarding the successive stages of the Ice Age in Applecross. First It is probable that before the extreme glaciation of that region these lofty mountains had their local glacier systems flowing from the high corries towards the sea-leveL Second. As the cold increased and the region became enveloped in a continuous sheet of ice, this local flow was arrested, and the Applecross peninsula was glaciated in one determinate direction towards the north-west. During this stage in the history of the period, erratics of deformed gneiss and schistose Torridon sand­ stone were transported from the ridge between Kishorn and Loch Carron, across the elevated plateau of Torridon sandstone to the shores of the Inner Sound of Eaasay. Third. When the mer de glace had declined or disappeared, local glaciers again supervened, bearing local boulders down the valleys to the shores of Loch Kishorn.