THE ICE AGE AND ITS WORK. 283 



The direction of tlie stria' and of the travelled bowlders, togctlier with 

 thi^ form of the great tenniiial moraines, show that there must "have 

 been two mai^i centers of outflow for the ice sheet, one over Labrador, 

 the other over the Laurentian Highlands north of Lake Superior. The 

 southern margin of the drift may be roughly represented by portions 

 of circles drawn from these two points as centers. The erratics on the 

 summit of Mount Washington show that the ice sheet must have been 

 a mile thick in its neighborliood, and much thicker at the centers of 

 dispersion, while the massesof drift and erratics on plateaus 2,(^^^*' ^^^^4^ 

 high near its southern boundary indicate a great thickness at the ter- 

 mination. The Laurentian plateau is now abont 2,000 feet above the 

 sea level, but there are numerous indications from buried river (dian- 

 nels, filled with drift and fiir below the sea, which lead to the conclu- 

 sion that during the Ice Age the land was nmch higher. That snow 

 can accnnudate to an enormous extent over land of moderate height 

 when the conditions are favorable for such an accumulation is shown 

 by the case of Greenland, the greater part of whose surface is a vast 

 plateau of ice flowing outward by numerous glaciers into the sea. The 

 center of this plateau where Dr. jSTansen crossed it was over l»,O00 feet 

 above sea-level, and it may be very much higher farther north. It 

 therefore seems prol)able that the great American ice sheet was at 

 least as high, and perhaps much liigher, and this would give sufficient 

 slope for the flow to the sonthern border. Of course, during the suc- 

 cessive stages of the glaciation there may have been numerons local 

 centeis from which glaciers radiated, and during the passing away of 

 the Ice Age these local glaciers would have left strijB and other indi- 

 cations of their presence. But so much of the area covered by the 

 drift — all in fact south of the New England mountains and the Great 

 Lakes — is nndulatiug ground, hill, valley, and plateau of modeiate 

 height; that here all the phenomena seem to be due to the great con- 

 fluent ice sheet during the various phases of its advanceand its passing 

 awny. - - - 



It will now be well briefly to sketch the distribution of erratic blocks 

 in Great Britain, and the conclusions to be drawn from them as to the 

 former existence of an ice sheet under which the greater part of our 

 islands was bnried. 



Every mountain group north of the Bristol Channel was a center 

 from which, in the earlier and later phases of the Ice Age, glaciers radi- 

 ated ; but many facts prove that during its maximum development these 

 separate glacier systems became confluent, and formed extensive ice 

 sheets, which overflowed into the Atlantic Ocean on the west and spread 

 far over the English lowlands on the east and south. This is indicated 

 l)artly by the great height at which glacial striie are found, reaching to 

 2,500 feet in the lake district and in Ireland, somewhat higher in North 

 Wales, and in Scotland to nearly 3,500 feet; but also by the extraordi- 

 nary distribution of erratic blocks, many of which can be traced to locali- 



