552 James Geikie — On Changes of Climate. 



valleys, the intercalated beds of the Till, although exceedingly 

 fragmentary, yet appear frequently (not always) to have, if I may 

 so express it, a marine aspect, I may just add, that intercalated 

 beds occur at all levels in our valleys, up to the highest limits 

 reached by the Till. 



From the facts thus briefly indicated, we are entitled to conclude that 

 that section of the Glacial epoch which is represented by the Scottish 

 Boulder-clay was not one long unbroken age of ice. It was certainly 

 interrupted by several intervening periods of less Arctic conditions, 

 during the prevalence of which the ice-sheet must gradually have 

 melted away from the low grounds, and given place to streams and 

 lakes and rivers. At such periods a vegetation like that of cold tem- 

 perate regions clothed the valleys with grasses and heaths, and the hill- 

 sides with pine and birch. Eeindeer wandered across the country, 

 while herds of the great ox and the mammoth frequented the grassy 

 vales. If one might draw conclusions from the aspect of the few fossil 

 remains which have been disinterred from the Boulder-clay deposits, 

 he might compare Scotland during the inter-Glacial periods to that 

 tract of country which extends along the extreme southern limits of 

 the "barren grounds" of North America — a region where a few firs 

 and other hardy trees cover the drier slopes, and where carices and 

 grasses grow luxuriantly enough in the sheltered valleys — those 

 favourite breeding-places of the reindeer which roam over the 

 dreary deserts to the north. Whether during any of our inter- 

 Glacial periods the climate was ever mild enough to melt 

 away all the ice and snow from our Highland valleys, the record 

 does not say. What evidence we have points to the existence of 

 local glaciers in our higher valleys — to moderate summers and 

 severe winters — during such inter-Glacial periods as we have any 

 certain records of. Nor must we forget the evidence supplied by 

 the mollusca of some inter- Glacial beds. At the time these shells 

 frequented our coasts, Scotland could hardly have had other than an 

 Arctic climate. 



And yet we might be committing a grave error were we to assume 

 that Scotland, during inter-Glacial times, never enjoyed milder con- 

 ditions than now obtain in the forest regions and barrens of North 

 America. We must ever bear in mind that the inter- Glacial deposits 

 are the veriest fragments. They have been preserved only in sheltered 

 hollows from the ravages of the great ice-plough, and the interrupted 

 and patchy portions that remain are mere wrecks of what must once 

 have been, in the broader valleys, widespread and continuous de- 

 posits. Every renewed descent of the glaciers upon the low ground 

 would tend to eifect the removal of these accumulations, and it may 

 well be that of many inter-Glacial periods not a single representative 

 deposit now remains. Even during the inter- Glacial periods them- 

 selves, the streams and rivers would help to clear away and redis- 

 tribute those beds of sand, gravel and silt which the glaciers had 

 spared ; just as in our own day the streams are gradually excavating 

 and washing away the materials which fill up old pre-Glacial and 

 inter- Glacial ravines and water-courses. Moreover, we must not 



