38 F. D. Chester—Gravels of Southern Delaware. 
ful. From numerous records of well-diggings, it is found that 
standing these difficulties there can be little doubt but that 
many of the representatives of the Delaware Gravels had their 
source in the older High Level formation. . 
laware Gravels.—Passing along the slope of the old Azoic 
hills, which trend ina northeast and southwest direction, be- 
1 
tween the Delaware and the Susquehanna rivers, a distinct 
and presenting throughout its entire length a more or less 
ragged outline, due to the subsequent decay of the hill-slopes 
upon which it rests. This shore line marked, as we know, the 
head of a Post-glacial estuary, into which the swollen Dela- 
ware poured its mass of debris. The deposit thus made, purely 
fluviatile in origin, yet spread over an estuary floor, we have 
called the Delaware gravels. It consists, as we have seen, of 
an upper layer of brick clay containing in most cases a large 
amount of fine and coarse gravel, cobble stones, and bowlders, 
with many features characteristic of the bowlder clay and hay- 
ing a uniform thickness of from two to three feet. Beneath 
this is a thick stratum of highly ferruginous sand and gravel, 
exhibiting eminent stratification, with oblique laminations, or 
flow-and-plunge structure, having an average depth of twenty- 
five feet. These Delaware gravels are spread out from the 
shore-line over all the northern part of the peninsula, of which 
a detailed description has already been given. 
Bog Clay.—By passing up and down the Delaware River, 
one notices at a variable distance of from one-quarter to @ 
