34() W. UPHAM — GLACIAL ACCUMULATION AND INVASION. 



there a much steeper gradient of the ice-front, and consequently more 

 vigorous action in the deposition of the general drift sheet and in amass- 

 sing marginal moraines than when the earlier drift was deposited. 



LOCAL ALPINE Olt DISTRICT ICE-SHEETS AND GLACIERS. 



In the Cordilleran mountain belt of the western United States many 

 large areas of alpine ice-sheets and glaciers have been found from the 

 boundary of the great northern sheet of glacial drift along distances of 

 from 700 to 800 miles south. Similarly the glaciation of northern 

 Europe was accompanied by a great extension of glaciers in the Alps, 

 the Caucasus belt, the Pyrenees, and other mountain tracts of France 

 and Spain. These areas had increased snowfall and their glaciers grew 

 by its accumulation, and the same conditions appear to have been suffi- 

 cient to produce the full extent of the contiguous continental ice-sheets. 



As the mountainous coast of Greenland has many local ice-caps of 

 only a few miles extent, with outflowing valley glaciers close to the 

 border of the inland ice-sheet, and as the zone of predominant wastage 

 by ablation on that ice-sheet reaches only 10 to 20 or 30 miles inward 

 from its edge, so I believe that the vast continental ice-sheets of Pleisto- 

 cene time grew by accumulating snowfall upon nearly all their expanse. 

 The Glacial epoch of their growth was soon succeeded by the Champlain 

 epoch of their departure, when the great Pleistocene winter ended with 

 a general change to a long secular springtime and rapid retreat of the 

 ice-sheets, scarcely less remarkable than their time of growth. 



PROBABLE CAUSES OF ICE ACCUMULATION AND DEPARTURE. 



Ice Accumulation attributed to high Land Elevation. — This conclusion, 

 that the ice came upon all its area principally by snowfall, is in accord 

 with the explanation of the causes of the Glacial period by great epeiro- 

 genic uplifts of the countries which became glaciated and drift-covered. 

 We need not here recount the evidences of the elevation of the drift- 

 bearing regions as high plateaus, raised above their present altitude from 

 2,000 to 3,000 feet or more, as known by the depth of submarine river 

 valleys on both our Atlantic and Pacific coasts and by the northern 

 fiords. The Sogne fiord, the longest and deepest in Norway, has a max- 

 imum sounding of 4,080 feet, and this probably measures approximately 

 the epeirogenic uplift, which at its culmination caused the envelopment 

 of some 2,000,000 square miles of northern Europe and the present ad- 

 joining sea beds by a vast sheet of land-ice. The altitude of the glaciated 

 areas of both these continents appears to have been sufficient to cause 

 their precipitation of moisture to be snow throughout the year, amassing 

 the ice on nearly the whole country which it covered with its drift. Only 



