122 
The American Geologist. 
August, 18et> 
When the Lehigh and Delaware rivers were just beginning 
to cut their water gaps, probably a stream, smaller than 
either of them, waf« cutting down the Wind gap. But the 
Lehigh and Delaware rivers, being larger, liatl an advantage 
over the Wind Gap river in that the}^ could lower their chan- 
nels through the mountain more raindly. When these two 
rivers had cut through the hard Medina sandstone and come 
into the softer Hudson River shales and Utica slates, they 
began lowering their beds at a greater rate; while the Wind 
Gaj) river was still slowly working away at the hard Medina 
sandstone. The relations of the outcrops of these f(»rmatif)ns 
are shown by figure 1. 
Fig. 1. Section along the line AA (plate III), showing- successive outcrops of strata, 
numbered according to the Pennsylvania Geological Survey. . 
Ill, Hudson River shales and Utica slates. IV, .Mcilina sandstone and Oneida con- 
glomerate. V, Clinton red shale. VI, Lower Hcldi rliiii;. VII, Cauda-galli grit and 
Oriskany sandstone. VIII, Chemung, Genesee, llauulton, Marcellus shales and Cor- 
niferous limestone. IX, Catskill red sandstone. X, Pocono sandstone. 
P. M., Pocono mountain. B. M., Blue mountain. W. R., Wire Ridge, capped by 
Catskill sandstone. P. Cr., Pohopoco creek. A. Cr., A<iuanchicola creek. 
The Lehigh and Delaware rivers, north of the Blue moun- 
tain, were at the baselevel of the bottoms of their gaps, as is 
shown by the broad valleys which extend down to the gaps 
and there suddenly contract only to widen out below. As the 
rivers lowered their beds at the gaps, the valleys of softer 
rock on the north would lower at nearly the same rate, thus 
giving increased activity to the tributaries which were cut- 
ting back toward the Wind Gap river. Since its bed was on 
a higher level, eventually the tributaries of either the Dela- 
ware or Lehigii tapped it and drew off all its water. Thus it 
can readily be seen how this gap was cut so long ago that 
there is now nothing left except the general form of the notch 
to tell of the former river. 
Yet there are some other indications that point out the 
general course of the ancient river as it flowed from the north 
to the gap. McMichael's creek has its source at a distance of 
about twelve miles northwest from the Wind gap; it rises 
farther north than any of the tributaries of Pohopoco creek, 
which flows west into the Lehigh river; and it flows in a 
southerly direction toward the Wind gap for some six miles 
