718 P. E. WRIGHT EFFECTS OF GLACIAL ACTION IN ICELAND 



the island being a barren waste of lava and glacial debris, devoid of 

 forests, and even vegetation, and worthless from an economic standpoint, 

 but valuable to the geologist because of its excellent exposures. 



Soundings by the Danish government have shown that these lowland 

 coastal areas, in north and west Iceland especially, extend seaward as a 

 coastal rock shelf 100 to 200 kilometers ont from the shore; in this shelf, 

 old valleys, 60 meters deep and of typical river valley features, have been 

 treated as continuations of the present fiords. According to Thoroddsen,^ 

 this continental shelf was cut in late Miocene or Pliocene, when all Ice- 

 land was uplifted; it was submerged at the close of the Pliocene. This 

 proof, that the present fiord valleys are old river valleys modified by ice 

 action during the Glacial epoch, is an important fact bearing on the 

 physiographic development of this region. 



During the Glacial epoch all Iceland was covered by an ice-sheet, which 

 moved from the center of the island oceanwards and profoundly altered 

 the physiographic aspect of the land surface. Eenmants of this ice-cap 

 still remain and occupy considerable areas in the interior plateau, 

 Vatnajokull covering over 8,500 square kilometers and Hofjokull and 

 Langjokull each about 1,300 square kilometers. 



Since the recession of the ice, the land surface has been altered very 

 slightly by water erosion, and, in north Iceland especially, has remained 

 practically as the ice left it. There the bedrock is the basalt formation, 

 which consists of a series, over 1,000 meters thick, of nearly horizontal 

 lava flows with intercalated tuff beds. This formation is fairly uniform 

 and free from elements which might tend to disturb the normal develop- 

 ment of the physiographic features characteristic of either ice or water 

 action; its material is, moreover, peculiarly adapted to retain the finer 

 markings produced by an erosive agent. Such a combination of fortu- 

 nate conditions is unusual, and renders this country specially suitable 

 for the detailed study of the land forms resulting from glacial sculpture. 



Two groups of such features of mountain glaciation, in particular, 

 attracted my attention during a short trip across the island in August, 

 1909; these were: (a) the glacial modeling and sculpture of preexisting 

 river valleys during the period of maximum ice development, or ice-flood 

 period, as E. C. Andrews and others have named it; (&) the effect of 

 the surface of the ice-sheet as a plane of reference toward which the up- 

 land surfaces tended to be beveled. My time was unfortunately limited, 

 and the impressions recorded in the following paragraphs are intended 



2 Island, Grundriss der Geographie und Geologie, von Prof. Dr. Th. Thoroddsen, 

 Peterman's Mitteilungen. Brganznngsheft. 152, 1906, pp. 93-98. 



