The Pre-Glacial Valley s of Arr'an and Snoivdon. 151 



III. The north-west to south-east valley across the north-east of the 

 island between Loch Ranza and Sannox Bay. 



IV. The valley which crosses from Brodick over a divide at 768 feet 

 and separates the mountains of North Arran from the moorlands that form 

 most of the southern part of the island. 



2. Evolution of the Existing Topography. 



The most striking feature of the major valleys of North Arran 

 is their independence alike of the main slopes and geological structure. 

 The major valleys are not radial from the mountains and they are 

 independent of the geological structure. Thus the three north and 

 south valleys cut right across the granite mass, and though their 

 lov/er ends bend seaward this change of course ajDpears independent 

 of the structure of the schistose and sedimentary girdle.' 



However greatly the valleys may have been enlarged by denuda- 

 tion, their plan was determined by some influence which aiiected 

 the area as a whole and was independent of its structure. 



In order to determine the origin of these valleys and the extent 

 to which they are due to glacial erosion it is necessary to determine 

 the general topography of the country at the beginning of its glacia- 

 tion. That topography was the result of the geographical evolution 

 of the island after the igneous activity of the middle Kainozoic. 

 The next process was the planing of Arran to a gently undulating 

 tableland, of which the surface in the Goatfell Mountains was, 

 perhaps, 2,000 feet above sea-level. The land extended further on 

 all sides, and the slopes were longer and gentler than they are now. 

 During this stage Arran was part of the mainland ; and, as Dr. Mort 

 suggests, the Chalmadale - North Sannox valley was probably 

 initiated then by rivers that flowed south-eastward across North- 

 Eastern Arran. The country was again uj)lifted, and Arran was 

 isolated by tectonic valleys to east and west, and a series of north 

 and south clefts parallel to these valleys cut across the granite 

 mass of North Arran. The clefts were subsequently enlarged by 

 denudation into the three major north and south valleys of the 

 island. 



After the second uplift the land must have remained stationary 

 for a sufficient period, for the formation of a low plain, which was 

 raised by a third movement into the thousand-foot platform 

 (Fig. 2). This uplift doubtless reopened the north and south clefts, 



■^ Amona; the valuable contributions in Dr. Mort's paper is his rejection of 

 the view that the difference in form between the central and western mountains 

 was due to the texture of the granite. His objections to that explanation 

 seem convincing. The western hills were completely overridden by ice, of 

 which the lower part would have been charged with rock debris from the 

 smooth central peaks and would have worn down the western summits into 

 rounded ridges. The lower part of the snow-cap over Cir Mhor and Goatfell 

 may have been converted to ice ; but after the ice had removed the loose 

 material it would have consisted of clean ice and have had but little abrasive 

 power. By the time it had reached the western hills the lower layers were 

 probably charged with rock fragments and were thus a more effective file. 



