SUPPLEMENTARY EEMAEK3 ON GLEN ROT. 



2. Supplementary Eejiarks on Glen Eoy. By T. F. Jamieson, 

 Esq., F.G.S. (Read November 11, 1891.) 



Contents. 



I. Conditions tliat preceded the formation of the Grlen Roy Lake — 



the Western Ice 5 



II. Level of th.e Snow-line 8 



III. Lake formed during the decay of the last Ice-sheet 9 



IV. End of the two Upper Lines in the Ordnance Map : Explanation 



simplified 9 



V. The Alluvium at Boh untine explained 11 



VI. The Ice-dam at the mouth of Grlen Spean _ 12 



VII. Subsidence of the Lake : traces of a debacle 13 



VIII. Mcol's objections answered 15 



IX. Height and horizontality of the ' Roads ' 17 



X. Beach-pebbles and Wave-action 19 



XI. Formation of the Terraces. Action of the Lake-margin 20 



XII. Dispersion of boulders from the Grlen Spean Syenite 23 



Appendix : 1. Prestwich's remarks on the Deltas 24 



2. His theory of the formation of the ' Roads ' 26 



I. Conditions that preceded the formation oe the 

 Glen Eoy Lake — the Western Ice. 



In order to have a right conception of the manner in which the 

 Glen Eoy Lake originated it is necessary to have some knowledge 

 of the state of matters that preceded it. During the Glacial period 

 the ice upon the West side of Scotland seems to have been vastlj'- 

 thicker than it was on the East, owing no doubt to the snowfall 

 having been much heavier in the former district, just as is now the 

 case with the rainfall there. The amount of rain in the West 

 Highlands is twice or three times what it is on the East coast. 

 This is due not so much to the greater height of the hills as to the 

 fact that the clouds of vapour coming from the Atlantic have most 

 of their moisture condensed there. ^ It is there that the wet sponge 

 is first squeezed. The rain-gauge on the top of Ben Nevis has 

 yielded no less than 145 inches of water in the course of a single 

 year, whereas at Laggan, in the upper part of Strath Spey, the 

 average is 46, and on the shores of the Moray Eirth only from 25 

 to 30 inches. It is also important to observe that it is during 

 winte?' that the greatest precipitation takes place. In some seasons 

 as much falls on Ben Nevis in the month of December alone as 

 there does near Inverness in the course of a whole year. 



We shall probably not be far wrong in assuming that during the 

 Glacial period most snow fell in Scotland where there is now most 

 rain, and if we consult the valuable maps which we owe to 

 Mr. Symons and Mr. Buchan we shall see that the wettest part of 

 the country lies along the region which extends from the mouth of 

 the Clyde to the Isle of Skye, passing right across the entrance to 



^ Quart. Journ. Geol. Soc. vol. xxx. (1874) p. 334. 



