the Middle Atlantic Slope. 887 



quently extend across the divides from river to river. Thus 

 between the Roanoke and the Appomattox the highways 

 traverse sensibly horizontal plains, only broken at long intervals 

 by the low steep scarps of terraces, or by the narrow steep- 

 sided, ravines incised within them, the plains being of such 

 extent as to yet remain imperfectly drained. The same is true 

 of the low plateau between the Appomattox and the James, 

 and of the greater part of the country between the James and 

 Rappahannock. North of the Rappahannock the high level 

 terrace-plains become less conspicuous as the shore line rises 

 from the friable elastics to the firm crystalline terranes, but 

 considerable plains occasionally occur ; and between the Schuyl- 

 kill and the terminal moraine beautiful terrace-plains of wide 

 extent are carved out of the Triassic sandstones up to altitudes 

 of nearly 200 feet. The terraces are best developed along the 

 inland margin of the Columbia formation, but reach somewhat 

 greater altitudes, ranging from about 100 feet in the south to 

 more than 400 feet in New Jersey. 



Though most prominent along the fall-line, the terraces are 

 not confined to it, but occur at intervals over the entire Coastal 

 plain, their extent and perfection depending upon the mate- 

 rials in which they are carved or from which they are built, 

 and upon the erosion they have suffered. Thus in eastern 

 Maryland it is possible to approximately map the western 

 boundary of the greensands by the perfection of the terraces 

 — the firm clays to the westward being everywhere distinctly 

 terraced, while the more friable sands have assumed the char- 

 acteristic undulating surface happily designated " topographic 

 old age " by Chamberlin. 



In brief, there is a practically continuous series of terraces 

 and beach marks along the fall line from the Roanoke to the 

 terminal moraine — a series of shore lines as distinctive and un- 

 mistakable as those circumscribing the valleys of the extinct 

 lakes of the Great Basin, of India, of northern Arabia, or of 

 the partially ice-bound basins of Minnesota, Michigan, Ohio 

 and New York, though they are generally more profoundly 

 modified by erosion and are frequently concealed by forests. 

 These shore lines embody an easily interpreted record of 

 geologic vicissitude which coincides in every detail with that of 

 the Columbia deposits. They are sometimes carved out of the 

 sub-terrane but are generally built of the loam, sand, and gravel 

 of which the Columbia formation consists, and are evidently 

 coeval therewith. Now it is manifest that these terraces are 

 water fashioned ; but they are not fluvial. There is always a 

 vertical component in fluvial action, and the energy of the 

 action varies with the value of thi& component ; every alter- 

 ation in the course of a wandering stream means change in 



