184 THE RIVER VALLEYS OF 



When rivers occur in flats bounded by cliffs, the 

 rivers are supposed to have formed the flats. In some 

 cases this may be right, but it is by no means neces- 

 sarily so, as the sea will sometimes do exactly 

 similar work in an estuary or fiord. The fiord called 

 Killary Harbour divides the County Mayo from the 

 County Galway. It is over ten miles long, and 

 usually less than half a mile wide. The five miles 

 toward the west bear N. 60 W. ; the four next miles 

 run nearly west and east, while the eastern part has 

 a general bearing of about N. 60 E. It is known 

 from the geology of the adjoining country, that the 

 gut occupied by the fiord coincides with different 

 lines of fault, and that each bay and turn in it are 

 also due to faults, some of these being of immediately 

 Post-silurian age, others of Post-carboniferous age 

 but pre-glacial, and some probably post-glacial. It 

 is also known that previous to its being occupied by 

 the sea, ice flowed down it. This is proved by 

 the presence of the moraine-drift that was left on 

 dressed, grooved, polished, and etched rock-surfaces, 

 which drift is now being excavated into and carried 

 away by the sea. 1 Since the ice occupied this 



glacier having occupied a similar gorge. On the north of these hills the 

 gravels of the Esker-sea are well developed. 



1 It has been said that because a glacier flowed down this valley prior 

 to the present occupation of it by the sea, the latter could not have 

 made it. This, however, does not follow, as the sea may have formed 

 it before the ice invaded the country, when the land was rising out of 

 the water during a pre-glacial elevation of the country. 



