ON THE SO-CALLED ''POSTGLACIAL FORMATIONS" OF 



SCOTLAND 



JAMES GEIKIE, 



University of Edinburgh 



The purpose of this paper is to pass in review the several geo- 

 graphical and chmatic changes which followed on the disappearance 

 of the last very considerable snow-fields and glaciers from the Scottish 

 mountains. Three well-marked stages in the glaciation of our 

 country are readily recognized. The earliest of these is represented 

 by the widespread ground-moraine long known here as our ''Lower 

 Bowlder Clay." This deposit was laid down during the epoch of 

 maximum glaciation, when the whole country — mainland and 

 islands alike — lay buried underneath one vast ice-sheet, which 

 extended south as far as the valley of the Thames. The succeeding 

 stage is evidenced by our "Upper Bowlder Clay"^ — a deposit of the 

 same general character and origin as the lower ground-moraine. 

 The ice-sheet underneath which it accumulated, however, did not 

 flow so far southward as its predecessor. It seems, indeed, to have 

 barely reached the midlands of England. Nevertheless, Scotland 

 was as broadly covered by this ice-sheet as by that of the earHer 

 epoch. The next stage of glaciation was marked by the presence 

 of district ice-sheets and large valley-glaciers in our Highlands and 

 Southern Uplands, and by less imposing snow-fields and glaciers 

 in the mountainous parts of England, Wales, and Ireland. This 

 stage is represented by local accumulations of bowlder- clay, terminal 

 moraines, and sheets of torrential gravels. 



There has always been considerable doubt among geologists 

 as to where we should draw the line between glacial and postglacial 

 deposits. Nor is this strange when we reflect that glacial condi- 

 tions must have lingered longer in some regions than in others. 

 The valley moraines of the Scottish Highlands, for example, belong 

 to a much later stage than the "chalky bowlder- clay " of the Thames 



I The substance of a lecture given to the Scottish Natural History Society, June 

 7, 1906. 



668 



