300 



GEOLOGY. 



beneath the end of the glacier. If the end of the glacier is retreating, 

 the retreat means that the waste at the end exceeds the forward move- 

 ment. If the ice advances 300 feet per year, and is melted back 500 feet 

 in the same time, all the debris carried by the 500 feet which has been 

 melted has been deposited, and largely in the narrow zone (200 feet) 

 from which the ice has receded. Even in this case, therefore, there is a 

 notable tendency to marginal accumulation. If the end of the glacier 

 is advancing 500 feet per year while it is being melted but 300 feet, all 

 the drift in the 300 feet melted has been deposited, and chiefly at or 



Fig. 276. — Embankment completed. Near the last. 



beneath the immediate margin of the ice. To the marginal and sub- 

 marginal accumulations made in this way, the material carried on the 

 ice is added whenever the ice is melted from beneath it. This addition 

 is sometimes considerable and sometimes meagre. If the edge of the 

 ice is without much fluctuation in position, the material dumped over 

 its end may take the form of a narrow ridge or bowlder-wall {Geschiehe- 

 wall). If a glacier pushes material in front of it, this, too, becO^fe 

 a part of the general terminal aggregation of drift. 



