THEORY OF THE ORIGIN OF THE DRIFT. 553 



and presumably as its effect, the whole northern part of the continent 

 was covered with a general ice-sheet, 10,000 feet thick over Canada, 

 6,000 feet over New England, and thinning southward. From this 

 Archasan area as a radiant the ice moved with slow, glacial motion 

 southeastward, southward, and southwestward over New England, New 

 York, Ohio, Illinois, Iowa, and Dakota, regardless of all but the great- 

 est inequalities — filling the valleys, sweeping over the mountain-tops, 

 and glaciating the whole surface in its course. Northward the sheet 

 perhaps extended to the poles, although it was thickest on the Ar- 

 chaean axis ; for there are evidences of a northward movement from 

 this axis in some places. Its eastward limit was beyond the present 

 coast-line ; its southern limit about 38° to 40° north latitude (Fig. 

 931). Even farther south, high mountain-ranges, like the Colorado 

 mountains, were ice-covered, and great glaciers streamed down their 

 flanks and left their moraines (Fig. 930). Along the New England 

 coast possibly the ice-sheet in many places ran into the sea and pro- 

 duced icebergs, but wherever the limit was on land, as in the interior 

 of the continent and in some places even on the eastern coast, it doubt- 

 less formed a terminal moraine, though this has been mostly washed 

 away by subsequent erosion. 



Terminal Moraines of the Ice-Sheet. — AVe have already seen that the 

 limit of the ice-sheet — where this was on land — was probably marked 

 by a moraine. Fragments of such a moraine have been found along 

 this limit, especially in its eastern part. Westward it has been mostly 

 washed away by the floods issuing from the melting and retreat- 

 ing ice-sheet. The extreme limit, therefore, in most places is best 

 shown by the presence of glaciation. In one way or another it may 

 be traced throughout its whole extent. Its most northeastern end is 

 found at Cape Cod; thence it goes southwest through Nantucket, 

 Martha's Vineyard, and Long Island ; thence through Northern New 

 Jersey, Northeastern Pennsylvania, touching the southern border of Sew 

 York ; thence southwest through Ohio to the Ohio Eiver, whose north- 

 ern border it follows to the Mississippi ; thence crossing the Mississippi 

 it follows the Missouri on its south side, and so northwestward through 

 Montana and into British America. This may be called the ice-sheet 

 boundary. 



After reaching this extreme limit, the ice-sheet retreated to, or 

 probably beyond, the Great Lakes, and then advanced again, but not so 

 far as before. This second and more recent advance is marked by a 

 very distinct and nearly continuous moraine of irregular, deeply-lobed 

 outline. In its eastern part this second ice-sheet moraine is coincident 

 with, or undistinguishable from, the first already described. But in 

 Ohio the two moraines part company; the second moraine, instead of 

 passing southward to the Ohio River, sweeps in a series of looping 



