Vol. 57.] ORIGIN OF THE DUNMAIL RAISE. 191 



moraines of the glaciers which issued from the Wythbiirn Yalley 

 on the one hand and the Greenburn Yalley on the other, while the 

 central portion is dotted with heaps of avalanche-moraine, — that 

 is to say, debris which rolled down the snow-slopes and was left 

 on the ground when the snow melted away. The Birkside Gill 

 shows old stream-gravels, washed down from the hills above, over- 

 lain by this avalanche-moraine material, indicating that the gap 

 was in existence before the Glacial Period. The sides of the gap, 

 too, where rock is exposed, show no signs of the ice-scouring which 

 should have been most conspicuous if the gap had been cut out 

 by ice ; this is especially well seen at the southern end of the gap, 

 where the spur between the Greenburn Yalley and that of the 

 Dunmail Eaise has been smoothed and rounded up to a certain 

 point by the glacier which overflowed from the former, and the 

 boundary between the glaciated and ungiaciated hillsides can be 

 traced running down in a slant to the great moraines which lie at 

 the southern entrance of the gap. 



All this shows that the formation of the gap of. the Dunmail 

 Raise cannot have been due to ice, for the reason that it was 

 in existence, in very much its present form, before the Ice Age 

 commenced. 



There remains, so far as I can see, but one other possible ex- 

 planation, namely, that the gap was formed by a river which once 

 flowed across the whole breadth of the Cumberland hills, whose 

 volume and power were sufficient to enable it for a long time to cut 

 down its channel as the hills were elevated, till, finally, whether 

 from an increase in the rate of elevation, or a decrease in the 

 volume and power of the river, or both combined, it was no longer 

 able to do so, the river-valley was split into two portions, and the 

 direction of flow of the upper waters reversed. In this way, and 

 this way only, does it seem possible to explain the peculiar features 

 of the gap which forms the Dunmail Raise. 



This being so, it is of interest to determine whether the river 

 flowed from north to south, or in the reverse direction, and here 

 there is little room for doubt. In the first place we have the 

 unequal slopes on either side of the summit-level : when a river- 

 valley is crossed by a symmetrical axis of elevation, which splits it 

 into two portions by the formation of a barrier across it and the 

 reversal of flow of the upper waters, the slope will be steeper on 

 what was the down-stream side than on what was originally the 

 up-stream side of the crest ; for, in the first case, the slope due to 

 deformation is increased, and in the second diminished, by the 

 original slope of the valley, l^ow, I find that the Ordnance Survey 

 gives the height above sea-level at the summit of the pass as 

 782 feet; at Wythburn, 9 furlongs to the north, measured in a 

 straight line, the level is 568 feet, giving a mean slope of 189 feet 

 per mile ; 7^ furlongs to the south of the summit the height of the 

 road is 550 feet, or a mean slope of 247 feet per mile. If, instead of 

 the heights on the road, those of the valley-bottoms are taken. 



