728 F. E. WEIGHT EFFECTS OF GLACIAL ACTION IN ICELAND 



sected, and uplifted, but it might serve to account for certain features of 

 land sculpture which on the peneplain theory have been found difficult 

 to explain satisfactorily. 



Summary 



The purpose of the foregoing pages has been primarily to direct atten- 

 tion to Iceland as an unusually favorable region for the study of the 

 effects of mountain glaciation, both of the valley glacier and continental 

 ice-sheet type. The fairly homogeneous and unaltered basalt formation 

 of north Iceland offered practically uniform resistance to glacial action, 

 and the forms developed therein by ice erosion are, therefore, simple and 

 characteristic. The difference between the action of ice currents in a 

 valley and the effect of the continental ice-sheet on the upland areas dur- 

 ing the ice-flood period is plainly visible in Iceland. 



An ice current in a valley is homologous to a river in its behavior ; but 

 in accord with the highly viscous or plastic state of ice, it acts as a blunt 

 tool of wide bearing surface, and tends to clear out all of the smaller 

 details of the river valley and to straighten and simplify its general 

 shape and aspect. 



The features produced by valley ice currents at the time of maximum 

 extension fall naturally into two groups: (1) Those produced by the 

 action of the valley ice current alone and undisturbed by tributary ice- 

 streams, as U-trough shape of cross-section, straightening of river valley 

 course, alignment of its sides, glacial grooves and markings along valley 

 sides and bottom, steep valley head, often with cirque termination, 

 etcetera; (2) those occurring at the junctions with tributary valleys, 

 and resulting from the combined action of the trunk ice current and the 

 disturbing tributary ice current. These features differ in character, 

 dependent on the size of the tributary and its angle of confluence, (a) 

 A small tributary ice-stream entering the main valley at any angle has 

 but slight effect on the trunk ice current; its valley is undercut and left 

 hanging, the elevation of its mouth above the main valley floor depending 

 somewhat on its size, (h) In the case of a large valley uniting with the 

 main valley at a wide angle and able practically to hold its own with the 

 trunk glacier, tremendous forces are brought to bear on the valley bottom 

 and sides, deep basin-like cuts far below baselevel are scoured out, while 

 the valley sides are cut back and rise abruptly as steep cliffs, often over- 

 hanging and showing the characteristic double cliff slopes. At the conflu- 

 ence of several large converging valleys, the trunk valley is widened, its 

 floor deeply eroded below baselevel, and its sides cut back until sheer cliffs 



