284 R. S. TARR RECENT ADVANCE OF GLACIERS IN ALASKA 



open Pacific along a coast where at present the glacier front is faced by 

 strips of alluvial fan and beach. 



Consideration of the future Condition 



In addition to the possible future advance of other glaciers, it is inter- 

 esting to consider what may be expected to follow on the advance of those 

 glaciers which have already been affected. On this point Galiano glacier 

 throws much light. If we are warranted in basing our conclusions on this 

 glacier, we may infer that in each case the wave of advance will soon die 

 out, that stagnation will follow, and that ablation will proceed to again 

 mantle the margins with a cloak of morainic debris. 



Much of the moraine formerly available for this mantle will have been 

 lost as a result of the breaking caused by the recent advance. Some of it 

 will have tumbled down into the crevasses, and much of it will have been 

 carried away by the newly born streams. The removal of this material 

 will leave less to blanket the ice and to prevent it from wasting away. For 

 this reason a more decided recession of the margin of the glaciers would 

 be expected in the immediate future. This more rapid recession may be 

 expected to be increased by a diminished supply of ice; for, since the 

 shaking down of quantities of snow in the supply ground has in places 

 removed the accumulation of many years, until such time as its place is 

 taken by additional snow accumulation, there should be a decided defi- 

 ciency in supply. One may therefore fairly predict a distinct repession 

 following the notable present advance. 



Geologic Effects 



The sudden breaking of extensive areas of glacier ice and the advance 

 of the broken glacier margins give rise to some very notable geologic 

 changes. By the advance in at least two instances, the course of good- 

 sized glacial streams has been completely altered. The cracking of the 

 glaciers opens to ablation an enormous area of ice hitherto protected by 

 debris or buried beneath the surface of the previously smooth glacier. 

 Our observations on Hay den glacier prove that in the last of July and 

 early August the level ice-surface is lowered 4 inches a day by ablation. 

 By the great increase of exposed ice the volume of water emerging from 

 the glaciers is quickly and enormously increased. The accumulated 

 moraine on the surface, and particularly along the glacier margins, fur- 

 nishes an immense quantity of debris ; so that not only is the volume of 

 the glacial streams greatly augmented, but their activity in transporting 

 and depositing sediment is increased many fold (plate 23). 



