88 The American Geologist. AuRust, looi 
composed of sand, gravel and liug'c boulders. The material as 
a whole, is coarser near the mouths of the gorges where the 
rivers leave the Piedmont Plateau to pass into the Coastal Plain 
than it is in the more remote portions of the delta. The inter- 
tluvial phase possesses no such regularity of bedding-, but is 
indiscriminately composed of clay, sand and gravel largely of 
local origin. These delta deposits may be identified in all the 
principle rivers of the Middle Atlantic slope and are particu- 
larly well develoDed in the valleys of the Potomac, Svisque- 
hanYia and Delaware. Due to the presence of these huge boul- 
ders, which were evidently ice-born and indicate a climate much 
colder than exists today in the same region, as well as to the 
fact that the Columbia, when traced northward, is found to 
pass under the terminal moraine, it was concluded that it was 
Ouartenary in age and belonged to the earlier glacial advance. 
These beds, since their deposition, have been raised and tilted 
so that they now lie higher in the regions to the north than they 
do further south. Their present elevation was found to be 
about 500 feet on the upper Susquehanna and 245 feet at its 
mouth; 400 feet on the upper Delaware; 145 feet on the Poto- 
mac ; 125 feet on the Rappahannock ; 100 feet on the James and 
75 feet on the Roanoke. 
Mr. McGee also noted certain well-defined terraces which 
were distributed over the entire region, and spoke of them as 
follows :'^ 
"There is a practically continuous series of terraces and 
beach marks along- the fall line from the Roanoke to the term- 
inal moraine — a series of shore lines as distinctive and unmis- 
takable as those circumscribing the valleys of the extinct lakes 
of the Great Basin, of India, of northern Arabia, or of the par- 
tially 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 de- 
posits. They are sometimes carved out of the sub-terrane but 
are grenerally built of the loam, sand and gra\^el of which the 
Columbia formation consists, and are evidently coeval there- 
with. Now it is evident that these terraces are water fash- 
*Ather.Jour. Sci., Vol. 35, 1888, pp. 387-388. 
