ON THE GLACIATION OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. 29 1 



the piling up of flood-borne ice and snow, mingled and covered with other debris, and 

 that in many cases these temporary constructions may have had a very important 

 influence in shaping the destinies of the drainage system. Before leaving this subject, 

 we would like to notice a particular effect, for which the reversed drainage of the 

 Aikengall Burn may fairly be held responsible. Just to the north of the entrance to 

 the pass, perched upon its western bank, there occur two or three dry valleys, which 

 there can be little doubt were used successively by the marginal stream at a time 

 when the waters of the latter still found an escape southwards through the Aikengall 

 valley. They now " hang" with respect to the latter, since, being streamless, they have 

 not shared in the opportunity of renewed erosion, which it attained through the partial 

 reversal of its drainage. 



After what has gone before, there would be no advantage in discussing the history 

 of the Dryburn and Borthwick channels in detail. It suffices to point out that the 

 streams responsible for corrom formation in these two cases are the How Burn and Gore 

 Water respectively. 



We may now bring this part of the discussion to a close by observing that the 

 operations of corrom formation are by no means restricted to glacial drainage systems. 

 The circumstances which so greatly favour their growth in the latter are likely to 

 be reduplicated in other situations. Wherever a gently graded valley lies open at its 

 head and no longer supplied with a sufficiency of water, there a corrom is likely to 

 appear ; and many a great beheaded valley must have found itself in just such plight, 

 and accepted a corrom as the watershed of its lower reaches. The permanence of the 

 capture thus involved can readily be predicted from analogy with the East Lothian 

 types, for the diversion initiated upon alluvium would, as it were, be fixed by becoming 

 imposed, through continued erosion, upon the rock framework of the country. 



It may be well to add here, since the term corrom is now being introduced for the 

 first time into scientific literature, that the definition relates to function and not to 

 origin ; it is on this account free from any speculative element. In the beautiful Glen 

 Tarbert example (Ardgour), whence the name has been derived, the delta has accumu- 

 lated on the surface of a rock watershed. A similar case has been described by Dr 

 Marr * for Dunmail Raise in the English Lake District. The Glen Tarbert and 

 Dunmail Raise deltas are corroms, since they serve as watersheds, quite irrespective of 

 the fact that they cannot be held directly responsible for their behaviour in this 

 respect. 



While corrom formation is favoured in the major glacial channels, another modifica- 

 tion has affected the less-marked grooves running along the hill slopes and roughly 

 following the contours. Their fate can readily be pictured : the more important side 

 streams have built out their cones into the channels, and thus have furnished themselves 

 with bridge-deltas by which they have won their way to the other side. Once across, 

 they have made good use of the slope and set to work eroding a channel for themselves, 



* hoc. cit., p. 143. 



