ON THE GLACIATION OF EAST LOTHIAN SOUTH OF THE OARLETON HILLS. 19 



the one at A, the other at B. It will also be observed that the top of the A channel lies 

 at a higher level than the bottom of the B channel, so that the latter cannot have been 

 excavated to its present depth when the former began to be cut. This indicates that 

 the two were not employed merely during a continuous retreat of the ice front, for 

 under such circumstances no channel could have begun to cut north of B until the ice 

 sheet had withdrawn beyond the 525-foot contour. It is evident, then, that these 

 two were cut, in part at least, during an advance of the ice sheet, so that if there is 

 reason to believe, from the state of their preservation, that they belong to the period 

 of retreat of the ice sheet at all, and not to the period of its growth, we are forced to 

 conclude that the advance which they indicate must have been of the nature of an 

 oscillation affecting the general retreat. 



In applying this line of argument it is necessary, in the first place, to compare only 

 valleys that are dry, so as to eliminate uncertainties due to post-glacial erosion. It will 

 be noticed that we here compare the top of A and the bottom of B. Post-glacial 

 change can only be supposed to act so as to lower the former, and, in the case of a dry 

 valley, to raise the latter so that if the top of A be still higher than the bottom of B we 

 may confidently believe that the difference was no less marked in glacial times. 



The oscillations of the East Lothian ice sheet have left several memorials of the 

 type described above. Black Law, a little to the west of the kamiform gravels of 

 Woodhall, furnishes an excellent example, for instance ; so also does the great Rammer 

 Cleugh channel behind Deuchrie Dod, # described by Professor Young, but in the last- 

 named case there is additional evidence showing that oscillations at this stage were 

 repeated more than once. In the first place, a high-level channel, as shown on the 

 general map, obviously furnished the original eastern continuation of this overflow 

 channel, and must have been blocked by a re-advance of the ice before the eastern portion 

 of the present course came to be employed. Then began the excavation of the magnifi- 

 cent gorge which is known as Rammer Cleugh. It is two hundred feet deep and cut 

 through solid rock, the finest example of a dry valley in the county. It is hardly 

 conceivable that the great ice sheet retreated regularly and slowly during the cutting of 

 this deep channel, since under such circumstances it only had a mile in all to travel 

 before presenting the marginal drainage with a fresh path, for escape, lying to the north 

 of the Dod. We cannot, however, point to any direct evidence indicating oscillations 

 at this stage ; but later, when the cleugh had been excavated to its present depth, a 

 re-advance is clearly indicated by a group of morainic mounds charged with great blocks 

 of red sandstone carried from the lowlands, which rest actually on the bottom of the 

 valley near its western extremity. It is interesting to note that while this line of 

 moraines was breached by a continuance of the drainage in an easterly direction, the 

 small stream which now occupies the gap flows in the reverse direction as a result of 

 subsequent corrom formation. 



The same kind of evidence is rendered available owing to records of underground 



* Pl. I. % 1. 



