THE CONTINENTAL GLACIERS OF POLAR REGIONS 279 



ment has been a retrograde one we live in a receding hemicycle 

 of glaciation. The earlier Greenland glacier has now receded so 

 as to expose large areas of 

 the former glacier pave- 

 ment. In places this 

 pavement is largely bare, 

 indicating a relatively rapid 

 retirement of the ice front, 

 but at all points at which 

 the ice margin was halted 

 there is now found a ridge 

 of unassorted rock mate- 

 rials which were dropped 

 by the ice as it melted (Fig. 



FIG. 306. Marginal moraine now forming at 

 the edge of Greenland inland ice, showing a 

 smooth rock pavement outside it. A small 

 lake with a partial covering of lake ice occu- 

 pies a hollow of this pavement (after von 

 Drygalski). 



306). Such ridges, com- 

 posed of the unassorted 

 materials described as till, 

 come to have a festooned arrangement largely concentric to the ice 

 margin, and are the marginal or terminal moraines (see Fig. 336, 

 p. 312). Marginal moraines, if of large dimensions, usually have a 

 hummocky surface, and are apt to be composed of rock fragments 

 of a wide range of size from rock flour (clay) to large bowlders 

 (plate 17 A), which may represent many types since they have 



been plucked by the glacier 

 or gathered in at its surface 

 from many widely separated 

 localities. 



As the glacier front retires 

 from the moraine which it 

 has built up, the water which 

 emerges from beneath the 

 ice is impounded behind the 

 new dam so as to form a 

 lake of crescentic outline 

 (Fig. 307). Such lakes are 

 particularly short-lived, for 

 the reason that the water 

 finds an outlet over the lowest point in the crest of the moraine 

 and easily cuts a gorge through the loose materials, thus draining 



FIG. 307. Small lake impounded between 

 the ice front and a moraine which it has 

 recently built. Greenland (after von Dry- 

 galski). 



