Vol. 56.] OF GLACIAL EROSION AT LOCH LOCHY. 203 



already stated, be many years before the denudation is sufficient to 

 be measured with such accuracy as to give definite results. If a 

 ravine of 50 feet deep has taken 10,000 }ears to cut, it has only 

 increased its depth at the rate of 6 inches in a century. Hence it 

 is evident that a very careful and accurate survey of some of the 

 ravines is essential, in order that small changes of contour may be 

 measured. 



EXPLANATION OF PLATE IX. 



View from Laggan Lock, on the Caledonian Canal, of the south-eastern side of 

 the valley in which the upper part of Loch Lochy lies. [R e P r °duced 

 frum photographs,] 



Discussion. 



Mr. Harker welcomed this paper as enforcing the importance of 

 ice-erosion in the Scottish Highlands. He had observed in Skye 

 evidence of the widening and deepening of a main valley by ice- 

 action, subsequently to the establishment of tributary valleys. He 

 thought, however, that to calculate the date of the Glacial Epoch 

 from the total post-Glacial erosion effected by a stream and its 

 present rate of erosion would not necessarily lead to a very trust- 

 ■worthy result, since the present rate cannot be confidently assumed 

 to represent the average rate throughout post-Glacial time. 



Mr. Barrow said that in mapping the Highlands he had met 

 many of these straight-sided valleys, and had noted that they coin- 

 cided with the direction of movement of the ice-sheet. Whenever 

 the valley made a sharp bend and no longer coincided with this 

 direction, the straight-sided character was lost. This was usually 

 as true of the minor as of the major valleys. In addition, the 

 faces of hills on fairly open ground presented a singularly uniform 

 slope, looking towards the direction from which the ico came. Their 

 reverse sides differed greatly in configuration, their form depending 

 mainly on the nature of the rocks of which they were composed. 



The Rev. Edwin Hill fully agreed with the Author that the 

 period elapsed since the Ice Age might not be long. The numerous 

 parallel channels were a natural consequence of the uniform slope* 

 as rain-drops run parallel down a sheet of glass. That this uniform 

 slope was a consequence of glacial action seemed to need more 

 evidence. Side-valleys opening at a high level on the sides of a 

 main valley did not seem conclusive proof of the lower part being 

 formed by ice. Such may be seen in mud-flats bordering the 

 channel of a tidal river ; nor is the difference of level in the channel 

 above and below Niagara due to ice. 



Prof. Seeley said that he had seen evidences of the small amount 

 of denudation since the west coast of Scotland was glaciated, in the 

 persistence of glacial grooving in exposed situations about Gairloch, 

 But nothing was more impressive than the condition of the Parallel 

 Roads of Glen Roy, which, though cut into by gnllies similar to those 



