ON THE GLACTATION OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. 27 



now dividing the valley into two is a fairly conspicuous mound * built out by a little 

 stream entering from Blackburnrig Wood ; it lies half a mile to the south-east of the 

 pre-glacial watershed, and serves well to illustrate the general rule that a drainage area 

 which has received water from a neighbour under conditions of glacial restraint has 

 often to pay back with some of its own when that restraint is removed. The explana- 

 tion given above clearly applies here, for the streams which continue along the route 

 previously taken by the glacial torrent have not succeeded in deepening their course, 

 while the Edmonds Dean Burn, which enters north of the corrom, but still south of the 



L--CORROM. W=P/7£- GLACIAL WATERSHED. 

 Fig. 5. 



pre-glacial watershed, has found a comparatively rapid drop by following the reverse 

 direction, and has thus been enabled to cut a gorge for itself through the old lake 

 deposits, which is fully a hundred feet in depth. A very conservative estimate of the 

 advantage gained can be arrived at by a consideration of the following figures : the 

 height of the corrom above sea-level is 396 feet, and this fall can be achieved in the 

 one direction in three miles, as contrasted with twelve miles in the other. 



Turning next to the highway which leads round behind Cocklaw Hill (fig. 6), we find 

 a similar instance of the removal of a pre-glacial watershed and the later formation of 

 ■a corrom further to the south. It is the Aikengall Burn which, entering the great 

 through valley about a quarter of a mile beyond the site of the original watershed, has 



* PI. III. fig. 2. 



