64 CHANNELS AND GLENS OF AYRSHIRE. 



over which the streams run. The rule appears to be, that the 

 older the formations are, the fewer are the post-glacial glens cut 

 in them, apparently for the reason that the older the formation 

 the deeper the pre-glacial glen would be, so that when the land 

 rose after the glacial depression the streams would be the more 

 easily guided back into their old hollows in the deeper-sided glens 

 than in the shallower-sided ones of the newer formations. 



Mouths of the Rivers. — That the land stood higher before the 

 glacial period than it does now is clearly shown by the western 

 Scottish Lochs which are just drowned valleys, as has been often 

 remarked by geologists ; the land, after the great depression 

 during glacial times — like a spring which has been bent beyond 

 the limits of its elasticity — not having recovered its former level. 

 The mouths of some of the Ayrshire valleys are still beneath the 

 level of the sea, such as the Noddle, Gogo, Garnock, Irvine, 

 Girvan, Stinchar and App. 



The Ayr and the Doon flow on rock within the limit of the 

 tide, and this makes me think that at these parts they have left 

 the lines of their old valleys ; but, without boring, it would be 

 mere conjecture to suggest where the filled-up glens may be. 



Physiography of Ayrshire before the Glacial Period. — During 

 the glacial period, it is perhaps not going beyond the mark to 

 state that an average of 40 feet of material was removed from the 

 surface of the country (there may have been much more), part of 

 it having been re-deposited on that surface, and the rest carried 

 off, mostly into the sea, in the shape of mud, etc. The drift- 

 deposits if spread equally over the whole shire would certainly 

 not reach to anything like 40 feet, perhaps to not more than 

 10 feet. What makes me take 40 feet as the average minimum 

 that has been removed (or shifted) is the fact that in America, 

 outside the glaciated area, the rocks, even granite and gneiss, have 

 been rotted by percolating acidulated water to a depth of 60 feet 

 (of course during the glacial period they had all that extra time 

 to rot). Now in Ayrshire, under thick drift (say more than 

 10 feet), the rocks are always found to be solid, showing that, 

 before the drift was deposited on them, all the rotten rock had 

 been removed, and there is no reason to doubt that it was any 

 thinner than 40 feet ; even since the glacial period some un- 

 protected rocks have been rotted to a depth of 20 feet, and 



