230 THE ICE AGE W NORTH AMERICA. 



mountains in elevation — say 1,000 feet above the present 

 height of the valley. 



The erosion in the valley, then, in excess of that along the 

 mountains has been mainly chemical, and at least a thousand 

 feet of limestone have been thus removed. A simple further 

 deduction shows that, accordingly, Nittany Valley has been 

 one million years in process of formation. 



The limestone erosion could not begin before the latter 

 stages of the Mesozoic era, possibly not before the Cenozoic 

 era, as sufficient time must have elapsed subsequent to the Car- 

 boniferous age to erode all formations of the Palaeozoic era 

 above the Trenton limestone. One million years seems not in- 

 consistent with other estimates of geological time. 



In view of such facts the advocate of glacial erosion can 

 not continue to maintain that ice is the chief agency in form- 

 ing the contour of continental areas ; but must grant that, 

 by reason of the great length of time during which water 

 has been about its work of corrosion and erosion, it is, with- 

 out doubt, the most important instrument in diversifying the 

 features of the earth's surface. Still, however short, by com- 

 parison, have been the periods of glacial action, no one can 

 study a glacier or a glaciated region without being deeply 

 impressed with the eroding and transporting power of mov- 

 ing ice. To get a full conception of its erosive power, one 

 must either get beneath it, or be able to calculate the force 

 of its movement from what he knows of the nature of ice. 

 Neither of these plans is altogether satisfactory, but each of 

 them is to some extent feasible. 



From the nature of the elements at work it has qxp** 

 generally been supposed that an advancing glacier would act 

 like a plow or scraper, greatly disturbing and modifying the 

 deposits over which it passed. But observation and a more 

 careful consideration of the qualities of ice have materially 

 modified these early impressions. From the fact that a 

 stream of ice moves faster at the top than at the bottom, it 

 follows that its action on underlying deposits is more like 

 that of a drag than like that of a plow. The rocky frag- 



