ON THE GLACIATION OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. 11 



to account for the origin of the anomalous drainage system becomes all the more obvious 

 in that the latter is so conspicuously unstable. 



We shall now pass on to the discussion of the modifications other than corrom 

 formation which may be attributed to the streams which at present occupy glacial 

 drainage channels ; and as a preliminary step we may allude to a peculiarity of the 

 original glacial channels which has proved itself of great importance in this connection. 

 They have as a rule been cut to a very gentle gradient, and an example occurs, between 

 Kidlaw and GrifFord, which one might more fitly describe as the dry valley of a sinuous canal 

 rather than that of a stream. The feature no doubt must be accounted for by the fact 

 that at certain intervals these channels were called upon to conduct away large masses 

 of water which were able to maintain a current even after they had degraded their beds 

 to the local base level. Already several of these valleys have been dammed to form 

 reservoirs or ornamental lakes, and the number is always on the increase ; a low dam 

 serves to impound a considerable body of water owing to the extremely gradual fall in 

 the valley bottom ; and in fact at Pressmennan* it has been found necessary to place a 

 dam at the head as well as at the foot of the lake. Small natural tarns and marshes 

 are also frequent along the course of the streamless valleys impounded behind screes 

 * or deltaic cones. 



The glacial drainage channels still occupied by streams can, as a rule, be readily 

 recognised even upon the map, since they form an integral part of the anomalous 

 drainage system of the county. The gorge in which Spott Burn lies, and which proved 

 so terrible an obstacle to the Scots in their attack upon Cromwell, furnishes an excellent 

 illustration within easy walking distance of Dunbar. The burn itself is of very con- 

 siderable volume, and as it winds along the broad bottom of the valley it may be seen 

 in many places cutting into the steep bluffs on either side. Here at first sight one 

 might reasonably expect to find evidence of well-marked deepening of the channel since 

 glacial times. But the actual phenomena of the case emphatically negative this 

 suggestion, while they enable us to recognise the modification for which the present 

 stream has been responsible. A little to the west of Spott the valley, as we follow it 

 up-stream, bifurcates,t or rather it is joined from the south by another valley of identical 

 type. The remarkable feature is that the direct continuation westwards of the Spott 

 valley is entirely streamless,^ for the whole volume of water is supplied by the valley 

 entering from the south. Examination of the dry valley proves that it owes its stream - 

 less condition entirely to natural causes, and it may confidently be regarded as an 

 overflow channel which has been deserted ever since the final retreat of the ice front 

 from this neighbourhood. It therefore furnishes a datum from which to reckon the 

 greatest possible vertical erosion accomplished by the Spott Burn in post-glacial times. 

 The dry valley enters flush with the bottom of the stream-containing valley, or at most 



* The same is being performed for Lammer Loch now that the latter is being extended to form a reservoir. 

 + PI. II. fig. 1. 

 I PI. I. fig. 2. 



