382 W. J. McGee — Three Formations of 



a particular specimen or section represents ; the differences are 

 limited to systematic variation in altitude and coarseness, to 

 variation in volume (which is proportional to that of the 

 streams on which the deposits occur), and to inconspicuous 

 variation in composition resulting from the incorporation of 

 (1) local materials on each river, and (2) rock-flour and other 

 glacial debris on the more northerly rivers. The deposits are 

 evidently contemporaneous and homogenetic ; the structure, 

 composition, geographic and hypsographic distribution, terrac- 

 ing, and other features of each independently proves that it is 

 a sub-estuarine delta formed during a brief period of land-sub- 

 mergence and refrigeration, increasing northward ; and the 

 relations of the various deltas to the terminal moraine and 

 other deposits prove that this period was long anterior to that 

 of the last ice-invasion. 



The Interfiuvial Phase. — Character and Distribution. — The 

 fan-shaped deltas flanking the Middle Atlantic slope rivers at 

 the fall-line attenuate down stream and toward their periphe- 

 ries, and either disappear in feather edges along ascending 

 slopes, or merge into a distinctive deposit by which the inter- 

 fluvial portion of the Coastal plain is generally mantled. This 

 deposit, unlike the complementary and more conspicuous one 

 developed only along the rivers, is variable in composition and 

 inconstant in structure, and has a wide range in hypsographic 

 distribution. Four leading structural types, ranging in alti- 

 tude from 100 feet in the south to 400 feet in the north down 

 to tide level, may be discriminated. 



1. As exposed in the terraces and shore lines in the vicinity 

 of the fall-line, the deposit consists of a heterogeneous and 

 irregularly bedded mass of sand, gravel and bowlders fringing 

 the terrace, and increasing in thickness from perhaps a foot or 

 two upon the terrace plain to five, ten, or fifteen feet along the 

 scarp ; the materials being predominantly local, and evidently 

 derived largely from contiguous portions of the terrace-plain 

 but intermingled with loam, pebbles and bowlders similar to 

 those of neighboring deltas. Similar accumulations occasion- 

 ally fill old ravines and other depressions in the formerly 

 irregular surfaces now smoothed into terrace-plains. This type 

 of the deposit is well exhibited in the scarp of an extensive 

 terrace near Washington (in a cutting on the Falls Church road), 

 and in the cuttings in the northeastern part of the same city 

 on Benning's road ; but such exposures are common, and those 

 observed and noted between the Roanoke and the Delaware 

 are numbered by scores. They frequently occur above the 

 maximum altitude of the fluvial phase of the formation in the 

 same latitude. 



2. A second type is exhibited only in northern New Jersey 



