ON THE GLACIATION OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. 15 



supplied with such a superabundance of material from west and south that even they 

 were unable to cope with it. Later, when this supply had become in part exhausted, 

 and the passes to the east had at the same time been deepened, the gravel-spread was 

 attacked and kamiform ridges produced. Tn fact, the latter are here nothing more than 

 the remnants left between successive glacial drainage channels. The disposition of the 

 channels in this case is such as to indicate that they were formed when a re-advance of 

 the ice sheet had carried it forward again over the gravels ; but there are equally 

 numerous cases where a wide stretch of sand and gravel has been dissected by glacial 

 drainage channels, for which such an interpretation is quite unnecessary. In imagina- 

 tion place an ice sheet abutting against the front of any hilly tract of country, and 

 suppose that it remain stationary for a long time in the same position. The governing 

 principle operative throughout will be the tendency of the obstructed drainage to 

 furnish itself with a suitably graded channel leading to the nearest available outlet. 

 Hollows and gentle slopes will be loaded with sands and gravels, while ridges, which 

 rise above the general level, will be breached by cross-cut channels : but, supposing the 

 process perfectly continuous, reach after reach, where at first sediments had accumulated, 

 would attain, as it were, its saturation point and, refusing further deposit, would hence- 

 forth enter upon a career of erosion. 



Under more natural circumstances further complications would be certain to arise : 

 thus we have already hinted several times at the oscillations which marked the with- 

 drawal of the ice sheet from the Lammermuirs ; it is possible, however, in a first broad 

 view, to neglect the results attributable to these oscillations, since they affect matters 

 of detail and not of general principle. But even then the account we have sketched 

 above is inadequate, since the opportunities for the destruction of sand and gravel 

 deposits formed in front of a stationary ice sheet are evidently far less numerous than 

 if the ice sheet were slowly and continuously retreating ; thus many a time a once- 

 continuous terrace must have become reduced to a mere collection of isolated remnants. 



We are now in a position to read the evidence afforded by the East Lothian sands 

 and gravels. A remarkable feature in their distribution is that they are for the most 

 part concentrated into two definite and quite distinct areas in which, as we shall 

 endeavour to show, they have accumulated under somewhat different conditions. The 

 first of these areas is a well-marked belt or zone of maximum accumulation extending 

 from Tynehead in the west right round the hills to Oldhamstocks in the east, while the 

 second includes what may be termed the coastal spread. 



Examination shows that the first of these two zones or belts is roughly contained 

 within limits set by the 700- and 500-foot contour lines respectively ; further, a study 

 of the mode of occurrence of these deposits in the field shows that many of the gaps 

 which break the continuity of the belt are certainly the result of erosion. In fact, it 

 seems tolerably clear that at one time a sand and gravel terrace, similar to that described 

 by Prof. Geikie * for the Strathavon Hills, near Glasgow, reached round about the 



* hoc. cit., p. 175, pi. iii. 



