LAKE PASSAIC. 543 
of Convent station the top of the plain is half a mile wide. From 
its upper edge at the moraine it slopes gently to the southwest, 
Ceclinine 20 wt} By Kear tn Woes loeNle senile, — Iolite whe ellis Gynt 
abruptly, with a slope which declines about 50 feet in 125 to 
150 yards. While no single exposure was found which showed 
the complete tripartite structure, exposures were seen which 
showed the several beds individually, and there can be little 
doubt that good exposures would reveal the three in vertical 
section. 
The line marking the junction of the gently sloping upper sur- 
face of the plain, with its abrupt front, marks the approximate 
water level at the time the plain was finished. But when it is 
desired to fix the water level exactly, or even within two or three 
feet, many inconsistencies seem to be involved if it be assumed 
that the line marking the change of slope corresponds accurately 
with the water level. There are several reasons for believing that 
it does not. (a) It has been found that even where the change 
of slope is most marked and can be accurately fixed, its height 
varies three, five, or even seven feet within comparatively short 
distances. (0) A consideration of the manner in which glacial 
delta plains are formed shows that the pronounced change of 
slope must mark the point where the bottom current of the run- 
ning water which was rolling the débris, so far lost its velocity 
that its load was dropped. It would seem, therefore, that the 
upper edge of the steep slope cannot be built nearer the surface 
of the lake than the depth of the current which, at that particular 
point, is supplying detritus. Since the depth and velocity of the 
various streams supplying detritus were different, and since a 
subaqueous overwash plain was built by a number of streams or 
by the distributaries from a single stream, all of which have 
various depths and velocities, the upper limit of the frontal slope 
should be of varying heights on adjoining lobes, and might even 
vary to some extent on the same lobe. We conclude, therefore, 
that the lake level was somewhat (perhaps sometimes several 
feet) higher than the upper edge of the highest ‘‘fronts,” 
that the water covered the outer and lower part of the gently 
and 
