I902. \Vright. — The Glacial Origin of Glcndoo. loi 



If this explanation of the waterfall is the true one, we should 

 expect at the point of entrance of the stream a rise in the floor 

 of the pass of some 20 or 30 feet, and there is no evidence of 

 anything of the kind. It is, however, quite possible, and even 

 probable, that the quantity of talus subsequentl)- deposited in 

 a steep-sided valley like this would almost entirely obliterate 

 such a feature 



It would appear therefore that this theory, although quite a 

 possible one, has many difficulties and does not seem to fit the 

 facts at all well. 



(3.) The third and remaining explanation is that the pass 

 was cut by the escape of ice-dammed waters flowing from Glen- 

 cruagh to Glencullen. or, perhaps vice versa. According to this 

 theory the waters of the lateral stream (the amount of erosion 

 due to which would be negligible during the comparatively 

 short time in which the pass must have been cut) originally 

 poured over its side as a cascade, which subsequently cut 

 backwards year by year, until it reached its present position 

 about 200 yards up the stream. 



This explanation would account for the straightness and 

 uniformity of the pass from end to end better than any other. 

 The fact that the floor of the pass dopes from the summit in 

 both directions mi^ht be urged as an objection. This, how- 

 ever, would be caused by the more recent erosion of the 

 streams which flow in either direction from it, and the 

 unchecked accumulation of detritus at the summit. 



If it were possible to obtain further proof to confirm the 

 evidence mentioned above as to the glacial theor}', a very 

 interesting piece of research would be possible. We might, 

 by sinking wires in the face of the fall, in the mar.ner suggested 

 and carried out at Melbourne by Mr. Brittlebank,^ and observing 

 the amount worn off" every year for a period of about 10 years 

 or so, estimate the rate of retreat of the waterfall. Hence, bv 

 measuring accurately the distance to the edge of the pass we 

 could deduce the time taken for the fall to retreat that distance 

 at its present rate. It is, however, proved by the coarseness 

 of the old river gravels and other evidence that, in the period 

 which has elapsed between the close of the Glacial Period and 



' C. C. Brittlebank. The Rate of Erosion of some River Valleys, 

 Ceol. Mag., 1900, 4th Sen, vol. vii., p. 320, 



