ON THE GLACIATION OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. 25 



IV. The Influence of Corroms and Bridge Deltas in Modifying 

 the Glacial Drainage System. 



We have already seen that the glacial drainage channels of East Lothian have 

 characteristically a very gentle gradient. This feature owes its origin to the magnitude 

 of the torrents which the channels were called upon to cope with during the intermittent 

 floods of summer-time. Now it frequently came to pass that a low-grade valley of this 

 type was established at one stage in the withdrawal of the ice sheet, to be left later on, 

 as the retreat continued, open at its head and deprived of its main supply of water. 

 Some little trickling stream might still perhaps wind its way along the bottom, but such 

 would obviously prove quite incapable of coping with the tributes of sand, gravel and 

 alluvium delivered by burns descending into the gorge on either hand. Deltaic cones 

 have been formed at the entrance of every side stream.* They are sufficiently 

 conspicuous to be easily recognisable, and resemble exactly well known instances in other 

 localities. t In the upland districts they furnish sites for the shepherds' cottages, and 

 include the only patches of land brought under cultivation. They not infrequently 

 obstruct the insignificant drainage of the main valley, and so give rise to small tarns 

 and marshy tracts situated in their rear. 



By way of illustration the number of cones which have been formed along the course 

 of Edmonds Dean was determined (see general map and fig. 5), and attention was paid 

 to the path traced out by each tributary stream in flowing down its delta. Thirteen 

 cones occur in a distance of two miles, and of these, six drain centrally, two down 

 stream and five up. The drainage directions here given refer only to what occurs on 

 the actual surface of the cones, for on reaching the bottom of the main valley all the 

 tributaries turn to flow south-east — the original direction followed by the glacial 

 drainage. The cone occurring furthest up stream has, however, very nearly raised 

 itself to the entrance level of the pass, and an inconsiderable addition to its height 

 would develop it into a corrom or delta watershed. 



The number of fully-formed and operative corroms modifying the glacial drainage 

 plan of the county is so great that an enumeration is impossible. To realise the 

 important role which some of these corroms play, one has only to seek out the great 

 highways which were established, under conditions of glacial restraint, to connect up 

 drainage areas previously distinct. In almost every case the striking feature here 

 is the very minor importance of the re-distribution of drainage permanently effected ; 

 the lines of communication temporarily employed are now blocked by the formation 

 of corroms which coincide roughly in position with the old pre-glacial watersheds. 

 Further, the slight interchange involved in this process has taken place, more often than 

 otherwise, in a direction contrary to that of the old glacial stream. The cause of this 



* PI. III. fig. 1. 



t J. E. Marr, Scientific Study of Scenery, second edition, p. 166 and pi. I. C. Russel, River Development, 

 p. 138. 



TRANS. ROY. SOC. EDIN., VOL. XLVI. PART I. (NO. 1). 4 



