30 PROFESSOR P. P. KENDALL AND MR E. B. BAILEY 



often removing all evidence of the manner in which the crossing was made. A capital 

 example of a bridge-delta occurs at Common House, rather more than two miles south- 

 south-west from Spott. Several small streams uniting at this point have well nigh 

 obliterated a portion of a strongly marked contour channel ; better still, however, they 

 have later set to work again to destroy the delta, and in the deep gashes that they have 

 cut one may recognise both the nature of the infilling material and the continuation, for 

 some little distance, of the rock walls of the original gorge. 



While examples of bridging by deltas can seldom be demonstrated, it is still 

 moderately certain that this operation is responsible in large measure for the reticulate 

 pattern of the East Lothian glacial drainage system. 



There is, however, another and distinct way in which a glacial drainage channel may 

 become segmented. Especially in the hilly districts, where pre-glacial features were 

 never obliterated, the major streams have in many cases continued their course straight 

 across glacial drainage channels, merely because the withdi'awal of the ice left an open 

 path in this direction. 



It is obvious, then, that contour channels are very liable to division. When once 

 this has been accomplished the next step is to provide each segment with its own 

 independent watershed. Here of course an opportunity for corrom formation is 

 afforded, and it is one that, in point of fact, is but seldom neglected. 



V. Evidence regarding the Level of the Sea at the time of the 



Retreat of the Ice Sheet. 



We have not made a special study of the intricate question of the relations of land 

 and sea at the close of the glacial period, but attacking the question from a new stand- 

 point certain results have been attained which we consider to be worth recording. 



In the first place, the phenomena which we have been describing are emphatically 

 those of the retreat of an ice sheet obstructing land drainage, and become meaningless if 

 viewed in the light of a submersion theory. In fact, to fix the upper limit which can be 

 assigned to the level of the sea at the time of the retreat of the ice sheet we need only 

 find the lower limit reached by the denudation effects of the land drainage when acting 

 under conditions of glacial constraint. Caution must naturally be exercised in the 

 choice of evidence upon which deductions of this character are to be based, and none 

 but " dry valleys " should be employed for the purpose. All the necessary requirements 

 were found in an exceedingly well marked glacial drainage channel cut in rock and 

 terminating near Ewe ford, slightly more than a mile south-west from Dunbar ; the little 

 water that flows through it merely trickles along a ditch which has been dug to drain 

 the marshes which still occupy portions of its bottom. We may confidently assert that 

 under natural conditions the valley would be streamless ; and it is obvious that so far 

 from having been overdeepened since its desertion by the marginal stream, all the 

 processes at work have tended to its obliteration. In spite of this, however, it only 



