96 April, 



THE GLACIAL ORIGIN OF GLKNDOO. 



BY W. B. WRIGHT, B.A. 



[Read before the Dublin Naturalists' Field Club, 19 November, 190;, 

 Hud published by permissiou of the Director of the Geological Survey.] 



The glen which is the subject of the present paper ought to 

 be well known to all residents in Dublin who love a good 

 tramp or a day among the hills. It is a steep-sided pass con- 

 necting the valleys of Glencruagh and Glencullen. The 

 slopes are densely wooded with larch and fir, and the road 

 winds backwards and forwards over the stream, giving 

 glimpses of the river below rushing over ledges of rock, and 

 plunging into deep brown pools of peat-coloured water. This 

 stream, which adds so much to the beauty of the spot, enters 

 the pass about 200 yards north of the summit, and flows down 

 into Dublin. It has its gathering ground on the slopes of 

 Cruagh and Glendoo Mountains, and attains considerable 

 volume before it emerges laterally into the pass. A much 

 smaller stream pours down the side of the pass at the summit, 

 and at times, especiall}^ when in flood, divides, one portion of 

 the water going to the Dublin side of the pass and the other 

 to the Glencullen side. In the 3^ear 1868 Professor Hennessy 

 called attention to this exceptional phenomenon of two streams 

 flowing in opposite directions from a common source.^ He 

 did not, however, attempt to appl}^ this to explain the cutting 

 of the pass ; indeed this question does not seem to have pre- 

 sented itself to him at all. This stream, although Professor 

 Hennessy mentions having seen it of considerable volume in 

 flood time, is a mere trickle compared to the lateral stream 

 mentioned above, and it is quite impossible to imagine the 

 pass to have been cut by it, especially as it has no valle}^ out- 

 side the pass, but merely pours down its side. 



Another fact of importance is that about 150 yards up the 

 lateral stream, from the point where it joins Glendoo, is a 

 waterfall about 12 feet in height, with several smaller falls 

 close above it, which probably at one time formed part of the 

 main fall, but which have separated from it owing to the 



• 1 Proc. R.I. A., vol. X., p. 335, and vol. i. (2ud series) p. 36. 



