374 W. J. McGee — Three Formations of 



its of the Potomac river is proved by those of the Susquehanna 

 and Delaware ; and since the bowlders of the lower Columbia 

 at Washington are fully twenty times as large and abundant 

 as those brought down in the spring freshets of to-day, the 

 diminution in temperature must have been considerable. 



In brief, it is evident that the Columbia formation within the 

 Washington amphitheater is a sub-estuarine delta deposited 

 when the sea rose at least 150 feet higher, and the temperature 

 was considerably lower, than to-day. 



The Deposits on the Susquehanna. — About it's locus of tran- 

 sition from fluvial to estuarine condition (for Chesapeake 

 bay is simply the estuarine portion of the river), the Susque- 

 hanna is flanked by an extensive bipartite deposit, the upper 

 member of which consists of loam with occasional disseminated 

 pebbles and small bowlders, while the lower is a great mass of 

 coarse sand and gravel interspersed with- large bowlders. The 

 distribution of the deposit, vertical and horizontal, is limited 

 by the 240-foot contour, and it is typically developed only be- 

 low the 120-foot contour ; the most abundant materials of 

 determinate source are bowlders from the Piedmont and Appa- 

 lachian regions, and well-worn quartzite pebbles from the sub- 

 jacent Potomac formation ; the entire area is extensively ter- 

 raced ; and in general the phenomena duplicate those of the 

 Potomac river. They are described in detail and fully illus- 

 trated elsewhere.* 



Certain minor differences between the Susquehanna and Po- 

 tomac deposits are noteworthy. The former reach far the 

 greater volume, the thickness being thrice and the area twice 

 as great as on the Potomac ; the bowlders of the lower mem- 

 ber are much larger — the largest being from 100 to 200 cubic 

 feet in dimensions, or full}'' three times as large as those found 

 on the Potomac' and 50 times as large as those now transported 

 into the bay in vernal ice-floes ; the materials of the upper 

 member are finer than on the Potomac, and consist in part of 

 mechanically divided but undecomposed carbonate of lime, 

 which either forms a calcareous cement or segregates into cal- 

 careous nodules resembling loess-kindchen ; indications of ice- 

 berg action are found in the deposits ; and a much larger pro- 

 portion of the pebbles and bowlders of friable rock are sharply 

 angular and evidently ice-transported. The resemblances be- 

 tween the deposits on the two rivers in structure and composi- 

 tion, in geographic and hypsographic distribution, and in all 

 other distinctive characters, indeed, prove that they are homo- 

 genetic — i. e., that the Columbia formation on the Susquehanna 



* " Notes on the Geology of the head of Chesapeake Bay," 7th An. Rep. U. S. 

 Geol. Survey (in press). 



