456 W. J. McGee — Three Formations of 



On the upper Susquehanna the post- Columbia and pre-moraine 

 erosion sufficed to excavate a valley from one to two miles 

 wide and 200 feet deep, while the post-moraine erosion has 

 scooped out a valley only a quarter of a mile wide and less 

 than 100 feet in average depth ; and similar relations obtain 

 on the upper Delaware. Along the fall-line the post- Columbia 

 erosion is measured by the gorges of the rivers between their 

 falls and their embouchures into estuaries, just as the post- 

 moraine erosion is measured by the gorges of the drift-covered 

 area ; and the Potomac and the Niagara are among the most sat- 

 isfactory of these chronometers. Now since the emergence of 

 the land from the Columbia ocean, the falls of the Potomac have 

 receded through an obdurate terrane from West Washington to 

 Great Falls, a distance of fifteen miles, while the Niagara, 

 under conditions favoring gorge excavations, has receded only 

 seven miles since the last ice-sheet withdrew beyond its lati- 

 tude ; and the contrast is still more striking in scores of other 

 cases. The difference is exemplified by the smaller streams as 

 well as by the larger, and equally by the minor topographic 

 configuration — the hydrography of the Columbia being ma- 

 ture, while that of the superimposed drift is nascent, and the 

 Columbia surfaces being everywhere deeply furrowed and of 

 ancient aspect, while the drift and Champlain surfaces are 

 relatively little touched by time. In short, when post-moraine 

 erosion is measured in yards, post-Columbia erosion must be 

 measured in rods. 



3. The moraine is seldom completely oxidized and lixiviated, 

 and its rocks are seldom disintegrated ; but where equally ex- 

 posed the Columbia deposits are profoundly oxidized, lixiv- 

 iated and ferruginated, and most of its non-siliceous rocks are 

 disintegrated, while the materials are frequently cemented not 

 only b} T ferruginous but also by siliceous and calcareous matter. 

 The widely diverse degrees of alteration in the two deposits 

 everywhere serves as a criterion by which they may be distin- 

 guished. 



In brief, the various phenomena of the Columbia formation 

 proves that while it represents an epoch of cold and submer- 

 gence, it is many times as old as the moraine-fringed drift by 

 which it is unconformably overlain. It is noteworthy, too, 

 that the volume of Columbia deposits- is several times greater 

 than the volume of corresponding deposits of the later ice- 

 epoch, indicating that the earlier refrigeration was much the 

 longer ; and it is equally noteworthy that the later drift over- 

 laps far upon the earlier aqueo-glacial deposits, indicating that 

 the later cold was the more intense. 



The general relations. — A presumably complete sequence of 

 Quaternary deposits and of the events they represent has been 



