GROUND-ICE WEDGES 



THE DOMINANT FORM OF GROUND-ICE ON THE NORTH 

 COAST OF ALASKA 



E. DE K. LEFFINGWELL 



It is a widely known fact that the ground in Arctic and sub- 

 Arctic regions is permanently frozen to a great depth, only the 

 upper few feet thawing in summer. Nearly all observers have 

 reported the presence of bodies of more or less clear ice underlying 

 the surface of the ground, usually immediately below the limit of 

 annual thawing. Ordinarily the ice is represented as existing in 

 horizontal beds of some thickness and lateral extent, but the obser- 

 vations of the writer upon the north shore of Alaska show that there, 

 at least, the ground-ice occurs chiefly in a network of vertical 

 wedges, surrounding isolated bodies of the tundra formation. 



Although this form of ice is the dominant one in the area studied, 

 it is not held that it is the only one, nor that the theory of its for- 

 mation will fit every case. It seems quite certain that there are 

 several different kinds of ground-ice, each one having originated in 

 a different way. 



During the summer of 19 14 several dozen photographs of the 

 ice were made, but most of them were damaged later by water, so 

 that the writer has to depend chiefly upon sketches which ^ were 

 often hastily made. Fortunately Mr. P. S. Smith, of the United 

 States Geological Survey, had, some years ago, secured photographs 

 of ground-ice on the Noatak River, and one of these photographs 

 illustrates the wedge-form ice which is the subject of this paper 

 (Figs. I, 2). 



The chief difficulty encountered in the study of the tundra for- 

 mation arises from faulty exposures. The ground being of material 

 only consolidated by frost, a short exposure to the summer air will 

 cause slumping and consequent masking of the details. It is only 

 where wave or river action has undermined the face of a bank, so 

 that large blocks break off, that good exposures are formed. As 



635 



