HEMAEKS ON GLEN HOY. 15 



coalesced with that of Glen Treig when they advanced into Glen 

 Spean. 



When the water dropped from the level of the lowest parallel it 

 seems to have drained away quietly — at all events at first, until it 

 sank out of upper Glen Roy — otherwise it would have carried away 

 the fine old deltas near Dalrioch. Many of the gravel terraces at 

 lower levels near the mouth of Glen Roy and along the Spean down 

 to Loch Lochy may have been caused by the outflowing water 

 when the lake was finally drained. Some of them which I examined 

 were not quite horizontal, but had a decided slope down the valley. 



YIII . JS'icol's objections answered. 



Prof. Nicol, who stoutly maintained to the last that the ' roads ' 

 were sea-beaches,^ urged that there are no well-defined river- 

 channels or ' notches ' at the cols, such as he says should have been 

 cut by the streams flowing out of the supposed lakes, and Prof. 

 Prestwich seems to be impressed with the importance of this objec- 

 tion. The point therefore demands some notice. 



These cols, we must remember, are situated on flat surfaces, 

 sometimes of considerable width; it was not, therefore, to be 

 expected that any deeply-cut channel or ' notch ' should occur in 

 such spots, for water issuing out of a lake over level ground has 

 no power to cut a trench. To do this it must first gain momentum : 

 the cutting power of a stream is governed by its velocity, and that 

 is determined by its gradient. A river passing over a wide flat 

 surface tends to accumulate sediment rather than to cut into its 

 bed ; it is only when it has gathered speed and come to a good 

 trot downhiU that it begins to cut ; and the rate of erosion will 

 then depend upon the hardness and tenacity of the materials over 

 which the water flows. 



The col at the top of Glen Gluoy has been described by Sir 

 Thomas Dick Lauder and Charles Darwin. To use the latter's own 

 words, it " is broad and very level.'*' Nicol himself says it " is flat 

 and marshy." It was not to be expected that a deep channel 

 should have been cut under such circumstances. The amount of 

 fall for the whole of the first mile is so gentle that Sir Thomas Dick 

 Lauder states it at " about twelve feet," or 1 in 440. However, 

 when we follow the stream down to Glen Turret, where the descent 

 becomes more rapid, we find ample evidence of erosion. A deep 

 narrow ravine has been cut, and also a huge deltoid mass of detritus 

 carried down into the head of the Glen Roy lake when it stood at 

 the mouth of Glen Turret during the time of the lowest line ; so 

 that the evidence here, I think, is all we could expect. 



The col leading from Glen Roy to Lochan Spey is also situated on 

 a wide level swamp. To quote JSTicol's own words, " the bottom is 

 broad and flat." ^ The gradient eastward to Lochan Spey, according 



^ ' On the Origin of the Parallel Roads of Glen Roy,' Quart. Journ. Geol. 

 Soc. vol. XXV. (1869) p. 282 ; ibid. vol. xxviii. (1872; p. 237. 

 2 3id. vol. XXV. (1869) p. 285. 



