ON THE GLACTATION OF EAST LOTHIAN SOUTH OF THE GARLETON HILLS. 13 



by the waters passing from the one natural drainage area to the other, and one of them 

 is of such importance as to deserve special notice. It is the Borthwick dry valley, 

 connecting the course of the Gore and Tyne waters, and crossed by the railway just 

 south of Tynehead station. The changes which have led to its abandonment will be 

 dealt with in a later section, so that we shall only notice here the advantages which it 

 owed long ago to its particular situation. The Mid Lothian and East Lothian basins are 

 separated by a line of undulating hills, constituting the Eoman camp ridge which 

 extends northwards approximately at right angles to the general trend of the margin of 

 the retreating ice sheet. The col at the southern extremity of the ridge is not well 

 marked, but once the Borthwick channel had been established at this ill-defined pass 

 and had been excavated to its present depth, the ice sheet had to retreat fully 2f miles 

 further to the north before disclosing any opening of lower altitude across the ridge. 

 The Borthwick channel was therefore employed for a protracted period during the 

 retreat, and at one time must have served to drain an important lake occupying a 

 portion of the Mid Lothian basin. 



We may now pass on for a while to consider briefly the deposits which were, 

 produced by the agency of the same constrained waters as cut out the drainage channels 

 just described. 



Great masses of sand and gravel encompass the Lammermuir Hills round about, and 

 have long been familiar to Scots geologists. Like other accumulations of their kind, 

 they appeared to the earlier workers as the memorials of great submergences. 

 Professor Young, # however, in his record of the distribution of the deposits, refrains 

 from offering any suggestion as to their mode of origin ; and later, Professor GEiKiE,t in 

 several brief references, rightly regards them as of fiuvio-glacial origin. 



The glacial age of the gravels is beyond question. They overlie and here and there 

 intercalate with the boulder clay. They everywhere contain erratic pebbles, most 

 conspicuous among which are fragments of coal assorted into special layers in accordance 

 with their low specific gravity, and finally their distribution makes it necessary to 

 believe that they formed in proximity to the retreating front of the ice sheet. 



Various factors have been concerned in their production, and we may mention the 

 following main sources from which they have derived their material. 



1. The hill country, owing to the melting of its local glaciers, the re-opening of its 

 normal lines of drainage, and the rapid waste of the boulder clay covering its steeper 

 slopes, supplied a very large proportion of the whole. Along the northern front of the 

 hills the contrast between the composition of the gravels and that of the boulder clay 

 is rendered very marked by this circumstance. The boulder clay resulted, as we have 

 seen, during a passage of the ice sheet from west to east, often from low ground to high, 

 while the gravels of this series travelled in large measure from south to north, and 

 always from high ground to low. Thus while the boulder clay, carried on to the 

 Silurian Uplands, is charged with the debris of the rocks of the central valley, the 



* Geology of East Lothian, 1866, p. G5. t Great Ice Age, 1894, p. 211. 



