I902. Wright. — The Glacial Origin of Glendoo. 99 



phenomenon would for obvious reasons be far more common. 

 In the first place melting would proceed more rapidlj^ near 

 the margin, owing to the dark-coloured hills absorbing sun- 

 heat better than the ice. The result of this would be that 

 the surface would slope towards the hills, and be lower at 

 the heads of the valleys than at their mouths, and such con- 

 ditions would clearly hinder drainage along the margin and 

 favour the formation of lakes. Secondl}^ this slope would 

 tend to make the surface waters drain to the edge of the ice, 

 and would thus give a far more copious supply than could be 

 caused by hill drainage onl}^ 



It is, perhaps, advisable to remark here that these lakes 

 would most likel3^ as a general rule, be ver\^ small, and formed 

 mainly in the lesser valleys lateral to the main ones, and it is 

 not intended to imply that such large valleys as Glenasmoel, 

 for instance, were ever filled with an unbroken sheet of ice- 

 dammed waters, though such a thing would be perfectly 

 possible. 



The result of such drainage over a col is to cut a steep-sided 

 notch through the ridge, its depth and completeness from 

 end to end depending both on the volume of escaping water 

 and the time during -which it was held up by the ice. 



Of gaps of this nature across ridges there are in the Dublin 

 Mountains numerous undoubted examples. The fact of their 

 being quite dry, except for a trickle of rain-water from their 

 sides, and in such a position as to preclude the possibility of 

 their being cut under existing drainage conditions, renders 

 their origin almost a matter of certainty. 



Such, however, is not the case with the pass of Glendoo, 

 for the lateral stream mentioned above is of sufficient volume 

 to have cut the pass provided it could be supposed to have 

 gone opposite ways at different times. 



Before proceeding to discuss the question, it is necessary to 

 point out that it is not the object of the present paper to prove 

 that Glendoo has been entirely cut by ice-dammed waters, 

 but rather to show that the evidence points to the fact that 

 such erosion was instrumental in deepening it. This restric- 

 tion is necessary because the contour of the hills shows that, 

 even before the present steep-sided gap was cut, there must 

 have been an exceptionally low pass over the hills just here, 



