DRAINAGE OF THE GLACIAL PERIOD. 325 



of it. The terraces on the East Branch of the Susquehanna are 

 much more marked than those upon the West Branch, the 

 explanation being that the East Branch lies almost wholly 

 within the glaciated area, while only a few of the minor tribu- 

 taries of the West Branch come down from it. But wherever 

 they do so come, as in the case of Pine, Lycoming, and Loy- 

 alsock Creeks, a limited amount of drift from the far north 

 is distributed along their banks, and deposited at their junc- 

 tion with the main branch of the river. The Lehigh and the 

 Delaware are likewise marked by high terraces containing 

 pebbles from the far north, while the Schuylkill River, which 

 lies just outside of the glaciated limit, has no such terraces. 

 Thus, both by the method of agreement and of difference, we 

 prove the connection of these terraces of the so-called " Ter- 

 race epoch " with the gorged and gravel-laden streams of the 

 great Ice age. 



Before speaking of the lines of glacial drainage farther 

 east, it will be profitable to direct our attention to another 

 class of closely connected facts confirming the theory of the 

 glacial origin of these terraces. The larger part of the ma- 

 terial contained in them is derived from the glacial deposits 

 over the region through which the several streams flow. The 

 hard fragments of granite, quartzite, and various metamorphic 

 rocks from the region of Lake Superior and the Canadian 

 highlands are eminently fitted to withstand abrasion, and can 

 be rolled by a torrent for long distances before being ground 

 to powder, while the softer sandstones and shales of the newer 

 geological formations would be comminuted by the attrition 

 of a comparatively few miles' travel. Hence it comes about 

 that the terraces of the Middle States are composed, in a pre- 

 dominant measure, of material brought over the w r ater-shed 

 by the ice from the far north, and spread broadcast over the 

 country, and thence collected by the streams of water and 

 rolled along as far to the south as there was force in the cur- 

 rent to move them, or through as great a distance as the 

 hardness of the material of which they are composed would 

 enable them to resist complete attrition. There is no more 



