GROWTH AND DECLINE OF PLEISTOCENE ICE-SHEETS. 29 



laid down as a gradually attenuated sheet, with neither morames nor 

 drumlins. After the Kansan or culminatmg stage the ice retreated and 

 the drift underwent much subaerial erosion and denudation during the 

 Aftonian interglacial stage. Renewed accumulation and growth of the 

 ice in the lowan stage is again represented by an overlying thin and 

 somewhat even sheet of drift near the limits of that glacial advance, 

 which mostly failed to reach the earlier boundary. 



Next came the Cham plain epoch of general depression of the ice-bur- 

 dened lands both east and west of the North Atlantic, when the ice again 

 retreated, but apparently at a much faster rate than before, wdth great 

 supplies of loess from the waters of its melting. Moderate reelevation of 

 the tracts earliest uncovered from the ice almost immediately ensued, 

 and a permanent uplift from the Champlain subsidence thence advanced 

 like a wave from the borders toward the center of each of these conti- 

 nental glaciated areas as fast as the ice front retreated. During this gla- 

 cial recession prominent moraines, in notable contrast with the smooth 

 and comparatively thin outer drift, were amassed in many irregular but 

 roughly parallel belts, where the front at successive times paused or re- 

 advanced under secular variations in the prevailingly temperate and even 

 warm climate by which, between the times of formation of the moraines, 

 the ice was rapidly melted away. Occasionalh^, too, the retreating ice 

 became much thickened near its boundary, giving the peculiar condi- 

 tions of accumulation of the drumlins, which are somewhat analogous 

 with the marginal moraines. The smooth but steep drumlins are sub- 

 glacial aggregations of till heaped near the mainly waning ice front, while 

 the rougher moraine hills are its marginal deposits. Both were depend- 

 ent on secular variations of the average climatic conditions during the 

 Champlain epoch, requiring a few years or decades of years for their for- 

 mation, according to their varying size, the abundance or scantiness of 

 the englacial drift on and near their areas, and the vigor or feebleness 

 of the glacial currents. 



Comparison of present Ice Action in Alaska and Greenland. 



The Malaspina ice-sheet in Alaska has been slowly retreating, like the 

 Muir glacier and others of that country, during the past hundred years 

 or probably much longer. On all its border for a width of a few miles, 

 now thinned perhaps to a quarter part or less of the earlier depth, the 

 waning ice is covered by its formerly englacial drift ; but, in that cold 

 climate, the glacial movement is so very slow that forest trees, with lux- 

 uriant undergrowth of shrubs, and many herbaceous flowering plants, 

 grow on this drift lying upon hundreds of feet of ice as revealed by stream 



