202 VV. UPIIAM — PLEISTOCENE AND PRESENT ICE-SHEETS. 



haps the most extensive and continuously preserved forest bed is that 

 described by McGee in northeastern Iowa, where he believes that it bears 

 testimony of a very long interglacial epoch, much exceeding the time 

 since the end of the Glacial period* But the drift-covered and forest- 

 clad borders of the Malaspina glacier show that this and other vegetal 

 deposits between beds of till might be formed by oscillations of the ice- 

 front, sometimes probably readvancing 100 or 200 miles, but in other 

 cases perhaps only a few miles, as may often have accompanied the for- 

 mation of terminal moraines. 



The Ice Age viewed as a continuous and geologically brief 



Period. 



General Consideration of the Question. — This comparison of the ice-sheets 

 of present and past times seems to me best accordant with a reference of 

 all our glacial drift to a single continuous epoch of glaciation, which, 

 though occupying probably 10,000 years or perhaps twice or thrice that 

 time, was yet brief in comparison with the duration of most, other recog- 

 nized geologic epochs. The outflow of the upper part of the Pleistocene 

 ice-sheets probably exceeded the currents of narrow alpine glaciers, but 

 was less than the advance of broad and deep polar glaciers which end 

 in the sea. For the journey of Pleistocene bowlders 1,000 miles in the 

 ice-sheet, somewhat less than 3,000 years would be required if the average 

 of the glacial currents was five feet per day. The amount of the glacial 

 erosion and of the drift, when compared with the erosion by the Muir 

 glacier, imply a short rather than a long duration of the Ice age. This 

 conclusion is further affirmed by the continuance of the same species of 

 the marine molluscan faunas from the beginning of the Glacial period 

 to its end and to the present day. 



In the light of these considerations, therefore, we are led to question 

 whether the generally accepted doctrine of duality or plurality of Pleisto- 

 cene epochs of glaciation, with long interglacial epochs, rests on a secure 

 basis. Under the stimulus of Dr James Croll's brilliant theory referring 

 the Ice age to astronomic causes, many European and American glacial- 

 ists have interpreted forest beds and other fossiliferous beds intercalated 

 between deposits of till as good evidence of long, mild epochs dividing 

 successive times of glaciation; but the partially forest-covered Malaspina 

 ice-sheet indicates, as I believe, that these beds record only temporary 

 oscillations of the ice boundary, not necessarily nor probably occupying 

 any long time. Through reliance on such evidence in Minnesota and 



* " The Pleistocene History of Northeastern Iowa," in the Eleventh Annual Report of the U. S, 

 Geol. Survey for 1889-90. 



