278 EARTH FEATURES AND THEIR MEANING 



parison with those nearer the bottom, so that the upper layers 

 override the lower as they would an obstruction. The englacial 

 drift is either for this reason or because of rock obstructions 

 brought to the surface, where it yields parallel ridges corresponding 

 in direction to the glacier margin. Such transverse surface mo- 

 raines are thus in many respects analogous to those which ap- 

 pear about the lower margins of scape colks. In contrast to the 

 longitudinal or medial surface moraines the materials of the trans- 

 verse moraines are more faceted and rounded they have been 

 abraded upon the glacier pavement. 



Melting upon the glacier margins in Greenland. During the 

 short but warm summer season, the margins of the Greenland ice 

 are subject to considerable losses through surface melting. When 

 the' uppermost ice layer has attained a temperature of 32 Fahren- 

 heit, melting begins and moves rapidly inward from the glacier 

 margin. In late spring the surface of the outer marginal zone is 

 saturated with water, and this zone of slush advances inward with 

 the season, but apparently never transgresses the inner border of 

 what we have generally referred to as the marginal zone of the ice 

 characterized by relatively steep slopes, crevasses, and nunataks. 

 Upon the ice within this zone are found streams large enough to be 

 designated as rivers and these are connected with pools, lakes, and 

 morasses. The dirt and rock fragments imbedded in the ice are 

 melted out in the lowering of the surface, so that late in the season 

 the ice presents a most dirty aspect. At the front of the great 

 mountain glaciers of Alaska, a more vigorous operation of the same 

 process has yielded a surface soil in which grow such rank forests 

 as entirely to mask the presence of the ice beneath. 



In addition to the visible streams upon the surface of the Green- 

 land ice, there are others which flow beneath and can be heard by 

 putting the ear to the surface. All surface streams eventually 

 encounter the marginal crevasses and plunge down in foaming 

 cascades, producing the well known " glacier wells " or " glacier 

 mills." The progress of the water is now throughout in tunnels 

 within the ice until it again makes its appearance at the glacier 

 margin. 



The marginal moraines. Study of both the Greenland and 

 Antarctic glaciers has shown that if we disregard the smaller and 

 short-period migrations of the ice front, the general later move- 



