POST-TERTIARY TOPOGRAPHY. 581 



Susquehanna crosses the Medina ridges above Harrisburg, the slopes are 

 covered with a talus of loose rocks, indicative of a recently renewed attack 

 on their bases by the river. 



It must have been chiefly w^ithin this brief cycle that the fall-line displace- 

 ment, as explained by McGee,* was initiated, for its effects are all young. 

 The falls produced by it where the rivers cross the reefs of uplifted hard 

 rocks are but little w^orn back, and the estuarine shores developed where 

 the coastal plain was depressed are as yet but little filled by deltas. The 

 displacement had an effect in turning the drainage westward on the penin- 

 sular areas between the rivers against the dip of the beds on which they 

 flow, and in New Jersey this effect is noticeable even on the Triassic belt 

 further inland than the fall-line is generally placed.f I am disposed to 

 account for this by an additional displacement west of the fall-line, accom- 

 panied by the characteristic tilting of the displaced block, so that its formerly 

 level surface now slants gently toward the northwest. In the case of small 

 streams this was sufficient to determine the direction of their flow ; but the 

 larger streams, such as the Karitan, maintained their courses against it. It 

 is worth noting that the displacement of this inner block and the tilting that 

 accompanied it are in the same sense as those that must be inferred in ac- 

 counting for the much more ancient faulted Triassic monocline of the New 

 Jersey belt. It may also be noted in tracing out our geographic homologies 

 that Long Island sound appears to correspond with the deflected parts of 

 the Delaware, Susquehanna, Potomac and other rivers, being enclosed on the 

 south by the retreating margin of the Cretaceous formation, and on the north 

 by the old crystalline peneplain, laid bare of the Cretaceous cover that prob- 

 ably once stretched over its margin. 



Further in this brief and imperfect cycle I shall not attempt to go. 

 Changes of more modern date are marked by forms of such moderate em- 

 phasis that they must be deciphered by field-work rather than by map-work. 

 The oscillations by w^hich the deeper channels of the Connecticut and Hud- 

 son were carved beyond the present coast-line, by which the yellow gravel 

 of New Jersey, the Columbia formation of McGee, was strewn over the low- 

 lands, reaching an extraordinary development in the southern states, the 

 latest changes indicated by the small estuarine mouths of many of the coastal 

 rivers — all these require delicate observation on the ground on a scale of 

 minuteness not reached by our maps, and of fullness not reached by the pub- 

 lished accounts of our coastal geology. 



* Seventh Ann. Rep. U. S. Geol. Surv., 1888, p. 616. 



t Geographic Development of Northern New Jersey: Proc. Bost. Soc. Nat. Hist., vol. XXIV, 1889, 

 p. 415. 



LXXXV— Bull. Geol. Soc. Am., Vol. 2, 1800. 



