924 HARRY FIELDING RE ID 



true only when the changes are small, or when an average is 

 taken over a long period. 



Form of the glacier s lower end. — Let us now give our attention 

 to the lower end of the dissipator. We have seen that the sur- 

 face slope at any point depends on the velocity and rate of melt- 

 ing at that point, and varies inversely with them. The general 

 form will, therefore, depend on the differential velocity. (We 

 are considering a portion of the glacier so small that we may 

 suppose the melting uniform ; though this would be somewhat 

 modified by the direction of the sun's rays.) The larger the 

 glacier, the greater in general will be the differential motion, 

 and the more rapidly will the slope diminish as we ascend from 

 the end. For very small glaciers the differential motion is small 

 and the slope remains steep. If anything should cause an abnor- 

 mally rapid melting of the lower layers, the natural slope could 

 not be retained, but the upper layers would advance over the 

 lower ones, project and break off. Professor Chamberlin' in his 

 excellent descriptions and illustrations of the glaciers of northern 

 Greenland has introduced us to just such forms. Fig. 3 is a sec- 

 tion taken from one of Professor Chamberlin's photographs of 

 Bryant glacier,' which is a good example of the type of glacier 

 we are now considering. It is about 140 feet thick at the end; 

 the lower layers, for something more than half its thickness, are 

 full of debris ; the upper ones are practically clear with an occa- 

 sional layer of debris. The terminus of the glacier inclines 

 slightly forward in the dirty lower layers, above which the clear 

 ice projects in one or two somewhat overhanging vertical steps 

 to the upper surface of the glacier, which slopes back from this 

 point according to the natural surface of an alpine glacier. 

 Wherever a debris layer occurs in the clear ice there is a reen- 

 trant groove in the cliff, and the layer above overhangs. Wher- 

 ever the layer of debris is interrupted so is the groove. 



All indications show a very slow motion, which is probably 

 entirely accounted for by the small thickness of the ice. 



' This Journal, Vols. II and III ; Bull. Geol. Soc. Amer., 1895, Vol. VI, 199-220. 

 ^Bull. Geol. Soc. Amer., 1895, Vol. VI, PL III, Fig. 2. 



