It is a widely known fact that the ground in Arctic and subArctic 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 observations 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 formation 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 1914 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 formation 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