26 PROFESSOR P. F. KENDALL AND MR E. B. BAILEY 



is not far to seek. Suppose that a natural drainage basin A is forced by glacial 

 obstruction to seek an outlet through a neighbouring basin B, and that a level-bottomed 

 gorge is thus established connecting the two. On further retreat of the ice sheet a 

 readier way of escape will likely present itself to the waters of the A basin, while those 

 of the B basin may still persist along the same route to the sea^as heretofore. Under 

 these circumstances it would be natural for a tributary of the B system to establish the 

 corrom blocking the highway, since thus it would share in the advantages obtained by 

 the A drainage. 



Four very notable illustrations are afforded in our district of the action of the 

 processes which we have outlined above. Thus there is the striking instance of the 

 railway pass between Cockburnspath * and Grant's House, which it will be remembered 

 was specially noticed by Sir Archibald Geikie in his description ; then, again, there 

 are the two great highways, in part occupied by the Aikengall and Thurston Burns 

 respectively, to which Professor Young directed attention, and which serve to isolate 

 in the one case Cocklaw Hill and in the other case Spott Hill from the main 

 mass of the Lammermuirs. Last of all, far away to the west there is the Borthwick 

 channel, which for so long a time connected the Mid Lothian and East Lothian 

 drainage areas ; as mentioned before, Dr Crampton, working independently, has 

 come to the same conclusion as ourselves with regard to the changes that have 

 brought about the desertion of this last-named channel, and his account will shortly 

 appear in the second edition of the Geological Survey Memoir dealing with the 

 Edinburgh district. 



Let us now take the first of our four principal examples, that of the railway pass 

 between Cockburnspath and Grant's House (fig. 5). The position of the pre-glacial water- 

 shed, which formerly broke the continuity of this valley, can readily be recognised just 

 at the point where the railway passes through a short tunnel to avoid taking an 

 awkward little bend to the west. On the two sides of this old watershed the valley 

 has passed through somewhat different stages of development. South of the col the 

 glacial waters, for long supplied by the channel of Edmonds Dean, had nothing to do 

 but deepen the old valley which served to conduct them towards the sea ; t but to the 

 north of the col they had to deposit layer upon layer of gravel, sand, and silt, and thus 

 choke the lake which formed in this position, while, when crossing the col itself, the 

 waters were always busy cutting and digging. So it came to pass that depositing in 

 one place and excavating in another, they never rested until they had graded the whole 

 to one uniform gentle slope inclining from north-west to south-east. But now the ice 

 retired and allowed the marginal drainage to escape round the north of the obstructing 

 ridge by the St Helen's Church channel, described by Sir Archibald Geikie, and the 

 great highway was abandoned which had cost so much labour to establish. The corrom 



* Pronounced Copperspeth. 



t A triangular mound near the southern exit of the tunnel is artificial, being composed of angular rock d^brify 

 with occasional brick fragments, top-dressed with sand and boulder clay to allow of planting. 



