The Frontier of the Carolinian Fauna in the 

 Lower Delaware Valley 



BY SPENCER TROTTER 



A well-defined rise of land trends along the western side of 

 the lower Delaware Valley in southeastern Pennsylvania. To 

 a casual observer it forms that final ridge of upland country 

 from which one gets an outlook over the flat expanse of the 

 coastal plain. To a geologist it marks the seaward border of 

 an ancient Appalachian land of worn-down crystalline rocks, 

 along ^he foot of which the Delaware River or some older body 

 of water has spread its flood-plain deposits of gravel, sand, and 

 clay at a comparatively recent period. Where the rivers, flow- 

 ing down from the Appalachian plateaus, break across this line 

 of demarcation between the older rocks and the recent alluvium, 

 their courses are beset with a series of rapids just before they 

 pass into the deep slow-moving currents of their lower reaches 

 — a feature so characteristic of these streams that it is recognized 

 by geologists as the "fall-line." This fall-line extends for a 

 long distance and marks a very definite and rather abrupt 

 change in the flow of the Delaware and its lower affluents and 

 of the Susquehanna and other Chesapeake rivers. It possesses 

 interest for the historian and the student of political geography 

 from the fact that its lower edge determines the upper limit of 

 navigable waters, at least in the Delaware and the head of 

 Chesapeake Bay, while its ''falls" have long been a more or 

 less considerable source of power. These two features have 

 played an important part in the location of a group of cities, ex- 

 tending from New York through Trenton, Philadelphia, Wil- 

 mington, Baltimore, Washington, Richmond, Raleigh, Colum- 

 bia, and Augusta to Macon, affording water-power for the earlier 

 industrial development of the region and, in the case of the 



(13) 



