RATE OF RECESSION 87 



TOPOGRAPHY 



The influence of topography was important. The direction of 

 striae and the position of recessional moraines indicate the fact, 

 and detailed mapping of the ice front by means of the clays 

 reveal it still better, as researches in Sweden have shown. The 

 rate of flow of the ice was rapid in valleys and basins, while it 

 was checked on highlands. In the marginal zone the distal parts 

 of the valleys were filled with protruding ice lobes, though dis- 

 charging of icebergs counterbalanced their formation. As the 

 ice edge withdrew, the lobes gradually disappeared, and, when 

 the ice front reached the northern, proximal end of the valley, 

 it had become concave. The flow of the ice, though now slower, 

 was yet more rapid than in the higher neighboring parts, but it 

 was more than counterbalanced by calving. 



PRECIPITATION 



A valuation of the influence of precipitation, or nourishment 

 of the ice, is particularly difficult, since this had much the same 

 effects as the temperature and may have co-operated with it in 

 the great advances, readvances, and recessions. For certain 

 local readvances of the ice, not due to topographic conditions, 

 precipitation may have played the leading part, since this is 

 likely to show greater differences between near-by regions, topo- 

 graphically similar, than does the temperature. The ice lobes 

 southwest of Lake Superior may be taken as examples. 



There seems to be one known case of a halt in the retreat in 

 Fenno-Scandia chiefly determined by increased precipitation, 

 namely that at the Inner Salpausselka in southern Finland. 

 While the ice edge halted for 183 years at this morainic line 

 (Sauramo, 191 8, pp. 35, 38, 43, 44), which in some districts, is 

 divided into a series of moraines, a clay consisting almost ex- 

 clusively of fine material was deposited, as usual during halts. 

 In contradiction to the rule, however, the varves marking the 

 halt are considerably thicker than those deposited during the 



