INTERGLACIAL EPOCHS. 



259 



bourhood in which our sections occur could have been over- 

 flowed. It is further made clear by the fact that the lower 

 till contains stones which could only have been derived 

 from the west. This till, therefore, is the bottom -moraine 

 of an ice -sheet which flowed from that quarter, and this 

 eastward motion of the ice in the vale of the Forth, near 

 Edinburgh, was determined, as I have already pointed out, by 

 the mutual pressure of the two ice-sheets which flowed from 

 the Highlands and Southern Uplands respectively. In point 

 of fact, then, the lower till of our sections proves that at the 

 time of its formation all Scotland lay buried in ice. The coarse 

 shingle which underlies it in one place belongs to the lower 

 till; it probably owes its origin to the action of sub -glacial 

 torrents, or it may represent some of the loose shingle which 

 was scattered over the face of the country before the ice-sheet 

 began to overflow that particular district. The beds resting 

 immediately on top of the till are of a somewhat variable 

 character, as will be gathered from the two sketch -sections. 

 In some places they show considerable false-bedding, and con- 

 sist of well-washed sand and gravel ; in other places they are 

 more or less earthy, and abound with angular and sub-angular 

 stones and boulders. They rest upon an eroded or worn surface 

 of the till, and are undoubtedly of aqueous origin ; and since 

 they perfectly resemble the deposits which are formed by streams 

 and torrents in the glacier- valleys of Switzerland and Norway, 

 they tell of a time when the ice-sheet melted away, and when 

 the waters derived from dissolving ice and snow washed up and 

 rearranged the old sub -glacial and other morainic materials 

 which the ice-sheet had left scattered over its deserted bed. 



2d, The next succeeding deposits bespeak changed condi- 

 tions. The flood-waters escaping from the retreating ice-sheet 

 had ceased to deluge the low grounds, and in the hollows of 

 the old glacial deposits appeared lakes, into which brooks 

 and streams carried fine silt and sand. By and by the climate 

 had become so far altered for the better that mosses and grasses 

 and birch -trees migrated slowly into the country, and insect- 



