Scottish Geographical Magazine ISSN: 0036-9225 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/rsgj19 Notes on the physiography of Arran Alexander Scott M.A. D.Sc. To cite this article: Alexander Scott M.A. D.Sc. (1918) Notes on the physiography of Arran, Scottish Geographical Magazine, 34:3, 90-99, DOI: 10.1080/14702541808555203 To link to this article: http://dx.doi.org/10.1080/14702541808555203 Published online: 30 Jan 2008. Submit your article to this journal Article views: 13 View related articles Citing articles: 2 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=rsgj19 Download by: [The University Of Melbourne Libraries] Date: 18 June 2016, At: 01:48 90 SCOTTISH GEOGRAPHICAL MAGAZINE. all its fortified lines. The battle of the Marne has often been described as a miracle; eyen the geographer may be excused for saying that it suggests that might lies not only in the sword and in the fort, but in something deeper. Turned back from Paris, the German armies estab- lished themselves on the Aisne, and to-day still hold Laoi~, on the edge of the westernmost barrier. The Argonne played its part in the battle of the Marne, despite its relative insignificance. On September 8, 1914, the Chief War Lord stood waiting, wrapped in his .martial cloak, we are told, to make that triumphant entry into Nancy which was to mark the conquest of the continuation of the rampart which runs north and south from Metz. That triumphant entry has been indefinitely postponed. The siege of Verdun was in part an attempt, due to the failure of the first offensive, to make that direct western attack on Paris from which the German staff shrank in August 1914. Summary as these notes are, they may serve to suggest the strategic value of the moderate elevations which guard Paris from the east, and are themselves due to differential erosion in the rocks of the Paris Basin and its margins. (To be continued.) NOTES ON THE PHYSIOGRAPHY OF ARRAN. By ALEXANDER SCOTT, M.A., D.Se. FROM the point of view of both geological structure and physiography the island of Arran may be divided into two parts of approximately equal area. The contrast in scenery is very pronounced, the northern portion consisting of a number of rugged peaks and ridges, rising to almost 3000 feet and interspersed with deep valleys; while the southern portion is mainly moorland with rounded hills which seldom exceed 1500 foot in height. The geological structure of the former has been described in detail by the Geological Survey (Gunn, 1903), Scottish Geographical Magazine 1918.34:90-99. and although no systematic description of the latter has appeared, several papers on the sedimentary and intrusive rocks have been pub- lished in recent years (Gregory, 1915 ; Tyrrell, 1915). Apart from the Glacial and Raised Beach deposits, the .youngest rocks in the island are the Cainozoic dykes and sills, and these are not later than Miocene. The northern half of the island consists of a great circular dome of granite surrounded on the north, west, and south by metamorphic rocks of pre-Cambrian age; while on the east it abuts transgressively on Palmozoie sediments. The mountains, all of which are found in the granite area, are divided into two groups by the long north and south depression which runs up Glen Easan Bioraeh from Loch Ranza and down Glen Iorsa, culminating at Loch Iorsa. The eastern group comprises a number of ridges radiating from Cir Mhor, one running north-north-west over Caisteal Abhait to Creag Dubh, one south-south-east to Goatfell, and one south-south-west over A' Chir and Beinn Tarsuinn to Beinn Nuis. Subsidiary ridges branch off from NOTES ON THE PHYSIOGRAP]~tY OF ARRAN, 91 each of these, as, for example, the Suidhe Fheargus ridge from The Castles, Cioch na h'Oighe, and Am Binnein from Goatfell, and Beinn a Chliabhain from A' Chir. The eastern group includes a curvilinear ridge which proceeds in a general northerly direction from Beinn Bharrain over Beinn Breac (2333 ft.)1 to Meall nan Damh, and another from Beinn JTarsuinn (1819 ft.) 1 to Meall 1V[hor. The area of high land is bounded on the north by the Chalmadale-Sannox depression between which and the coast is a range of tow hills: On the southern edge is a broad plateau, about 1000 feet above sea level, and terminating in the through valley which carries the "String Road." The hills of the southern portion only attain a maximum height of under 1700 feet, and may also be divided into two groups. Situated immediately to the south of the String Road is a large Cainozoie vol- canic vent, the igneous and pyroclastie material of which gives rise to the Ard Bheinn group of hills, while to the south-east, and separated from the latter by another depression, is the Tighvoin group. This is formed by a large series of Cainozoic sills which have been intruded into the Triassic sediments. The contrast between the two types of scenery may be paralleled, to some extent, by a similar contrast which is found in Skye, where the mountainous region of the Cuillins is sculptured out of a great gab- bro laecolite, while the monotonous plateaux to the north-west are made up of the Cainozoie sills and dykes. The two districts present several other analogies, which will be discussed later. In endeavouring to determine the history of the topography of Arran, it is necessary to take into consideration, firstly, the effect of the dissection of the ancient Highland peneplain, and of the denudation which occurred prior to the separation of Arran from the mainland ; and, secondly, the effect of the denudation which has taken place since the separation of the island. It is generally recognised that the Scot- tish Highlands have been carved out of a plateau, which may have originally been a peneplain, and the present average height of which is Scottish Geographical Magazine 1918.34:90-99. between 2000 and 3000 feet. This tableland sloped gently from north-west to south-east, and many of the higher peaks of the High- lands stood out as monadnocks in the undulating plateau. It prohably extended much farther to the north-west than the present land surface, and many of the north-west south-east through valleys of the Highlands were initiated by the consequent streams which drained this plateau. Superimposed on this drainage system is a series of subsequent valleys with a general north-east and south-west trend, following the direc- tion of the strike of the Dalradian rocks, and parallel to a series of faults. In the south-west Highlands the trend is rather south-south- west. According to Peach and Horne (1910), the second base-level of erosion occurs approximately at the 1000-feet level, but these writers 1 As there are several hills of these names in the island, the heights are inserted in order to indicate which is referred to. 92 8(DTTISH GEOGRAPttlCAL MAGAZINE, give few details. Various high-level platforms have been described from the south-west of England and the Snowdon district. More ([914) has discussed the origin of the 1000-feet platforfn in Arran, but the first writer to describe the occurrence of such a platform in Arran was Smith (1896), who remarks upon " the immense amount of marine denudation which has taken place . when the sea worked between the 600-feet and the 1.300-feet contour lines." A third base-level probably occurs at some depth below sea-level, but the present level of this is variable owing to the submergence being differential. So far as Arran is concerned, there is little residual trace of the Highland peneplain. The mountains of the granite area probably existed as rounded hills, rising some distance above the general level, but all other traces have tong vanished owing to the enormous amount of denudation which has occurred since the uplift of the plateau. The 1000-foot platform, however, is very obvious, and can be seen at many places round the margins of the granite, and also some considerable way up Glen Iorsa. A detailed description of its extent in the northern half of the island is given by Mort (1914). In the southern area the platform is not so obvious, but in many places the land rises steeply to about 800 to 900 feet, and then a fairly abrupt change in gradient takes place, the slopes above 1000 feet being very gentle. This is clearly seen along the eastern margin of the Vent, while in the south- eastern corner of the island a broad extent of moor at the same level is found. There is no very definite evidence as to the age and origin of the piatform. The generally flat surface suggests a high degree of maturity, while the small amount of dissection which it shows in the granite area --save where cut by the deep glens--is indicative of a comparative recent uplift. The comparatively steep gradients by which the moun- tains of the interior rise from the surface of the platform are favourable to the marine origin of the latter, but, on the other hand, the general structure of the southern tableland points to an advanced stage of sub~erial denudation. The absence of shore deposits, cliffs, and so forth, Scottish Geographical Magazine 1918.34:90-99. as pointed out by Mort, cannot be considered strong evidence in favour of the latter, as the platform was covered by the main ice-sheet of the Clyde area, and probably modified to a certain extent.