SUMMARY 729 



result. The junction spurs rise almost perpendicularly from their bases 

 to high rounded domes, often beehive-shaped, and impressive because of 

 their massiveness and simplicity, (c) At the confluence of a large valley 

 at an acute angle with the main valley, the junction spur, between the 

 two valleys, coincides closely with the direction of the ice currents. Its 

 sides are truncated and aligned, but its crest line is worn down by the 

 overriding, merging ice currents to a low, gently sloping tongue of bed- 

 rock pointing downstream. This type of spur is a characteristic feature 

 of many glacial valleys, and for it the name of glacier junction spur is 

 proposed. At and below these acute angled junctions, deep basins are 

 also usually cut by the confluent ice-streams. At all such large junctions 

 congestion of the ice may occur, and the uniform erosive action of the 

 trunk ice-stream may be disturbed, with resultant irregularities in the 

 valley rock floor. Later, on recession of the ice, irregularly shaped gravel 

 deposits may be formed at such points. 



These physiographic features which are characteristic of the action of 

 an ice current indicate clearly that it erodes most rapidly along the valley 

 floor and the lower half of the valley walls ; there the thickness and mo- 

 mentum of the ice are at a maximum. Higher up the sides of the valley 

 and nearer the original surface of the ice, the amount of direct erosive 

 work accomplished by the ice current decreases rapidly, and as a result 

 glacial valleys are usually steepwalled. 



In contrast to an ice current, an ice-cap is homologous in its action to 

 that of a large water surface, as the ocean, toward which exposed high- 

 land masses tend rapidly to be beveled. The ice-sheet spreads over wide 

 areas; its surface is fairly uniform and slopes gently from the center of 

 the country oceanwards and away from the elevated land masses which 

 support it. As a whole, the ice-sheet lacks the directive influence of the 

 valley which controls the ice current and renders possible the concentra- 

 tion of the tremendous forces of erosion which are dominant during the 

 flood period of the glacial ice. As a consequence, the physiographic fea- 

 tures developed by the ice-sheet are noticeably different from those result- 

 ing from ice-current action, even though such currents may be in part 

 simply submerged portions of the ice-sheet itself passing through former 

 river valleys. Immediately below the surface of the ice-sheet but little 

 erosion is accomplished, as the ice there lacks sufficient thickness to be 

 effective. Its attack, like that of a body of water, is directed chiefly 

 toward the margins of exposed land masses. These elevated areas are, 

 furthermore, subjected to the forces of highland erosion in its most viru- 

 lent form, and soon break down and disappear beneath the ice cover. 



