W. B. WrigJit—A Theory of the Marginal Drift. 553 



by Mr. Lamplugh, and the corresponding eastward shifting in Nortli 

 America. Mr. Lamplugh expressed the idea by saying : ^ "As the 

 plateaus of ice rose higher in the path of the moisture-laden air- 

 currents they must have gained increased effectiveness as condensers, 

 thereby not only augmenting the snowfall in one quarter, but also 

 diminishing the precipitation in the region to leeward." It might 

 be more in conformity with modern ideas of the meteorology of 

 ice-sheets to say that they drew down upon their surface the bulk 

 of the moist air of the uj^per current which was passing across the 

 ice margin towards the interior in virtue of the anticyclone 

 circulation . 



The Limiting Factors. — It is clear from what has been said above 

 that, as an ice-sheet expands, its central area becomes starved, active 

 precipiitation being confined to the peripheral areas. Even in these 

 latter, however, area for area there is less nourishment, as each 

 sector of the front has a progressively narrower sector of the extra- 

 glacial region to draw upon for its moisture. This is the first factor 

 tending to set a limit to the growth of an ice-sheet, and it is to be 

 noted that it is independent of any amelioration of climate. 



The second and perhaps the more important factor is the increased 

 wastage consequent on expansion. This is, of course, obvious if the 

 ice-sheet is thrusting forward into the zone of ablation, but even if 

 this zone is never reached there must ultimately come a limit. With 

 increase in size and power there is drawn down the central pipe of 

 the anticyclone a continually greater proportion of dry air from the 

 upper regions of the atmosphere, and this spreading radially over the 

 surface of the ice causes ever-increasing evaporation. Towards 

 the periphery it no doubt partly mixes with, but also tends on 

 account of its great weight to wedge in below, the moisture-bearing 

 air descending in the zone of nourishment. The evaporation effect 

 demonstrated by Sauramo to be a working factor in the colder stages 

 of the Finnish retreat may thus become a really powerful agent of 

 control during the rigour of the glacial maximum. 



Effect of Accumulated Gradient and, the Radial Snow-drift. — ^.Thus, 

 with failing nourishment and increasing Avastage by evaporation,, 

 an ice- sheet may reach a limit of growth without ever extending into 

 the zone of ablation. It is now losing by evaporation as much as it 

 gains by precipitation. Its powers of expansion, however, do not 

 stop here, for it has accumulated a gradient, and must consequently 

 extend its frontiers until the gradient becomes so low as to be 

 inoperative. In doing so it still further decreases its noxirishment 

 and increases its evaporation, so that the latter becomes dominant. 

 It is already wasting from above downwards where it reaches its 

 maximum extent, and this wasting continues until the front is again 

 withdrawn to the line of balance, where precipitation and evaporation 

 are equal. Here it awaits an amelioration of climate before ablation 

 can set in and further withdrawal take place. 



^ Pres. Address to Section C, British Association; York, 1906, p. 14. 



