PPvEGLACIAL SURFACES. 295 



direction through what was probably one of the many slight faults that 

 are found at the gap, while the sand dam probably always existed 

 where it is, owing to the fact that the Lehigh and Aquanchicola meet 

 " head on,'' and the greater force of the Lehigh would neutralize the 

 smaller stream and form a bar across its mouth, so that it would be de- 

 flected to the south and widen the crack made by a probable fault. 

 However that may be, the channel is as it existed during glacial times. 

 From Whitehall southward, along the Lehigh, both till and clay are 

 frequently carried to and below the flood-plain deposits, and at West 

 Bethlehem the clay extends continuously down the hill to the river. 

 The flood-plain is covered with a thick, unstratified gravel, through 

 which the railroads on both sides of the river have deep cuts. The new 

 well for Bethlehem, at the level of the Manokisy, passes through 13 feet 

 of gravel before striking limestone. Preliminary piling to settle the loca- 

 tion of the 120-ton hammer of the Bethlehem Iron Company on what 

 was a part of the old river bed of the Lehigh showed that the solid rock, 

 or what resisted as such, was 26 feet deep, on the average, over an area 

 of 75 feet by 75 feet. Subsequent results — a sinking of four feet — after 

 placing the hammer showed that solid rock had not been struck, and 

 excavations for various purposes developed the following section : 



River gravels — a few inches to 14 feet. 



Packer clay — 3 to 4 feet. 



Gravel (till) — bottom not reached, owin^ to influx of water. 



The piles struck bowlders in sinking, and some of them were driven e36 

 feet. We have, therefore, a buried glacial or preglacial channel, instead 

 of an eroded one, and. as before stated, this is along that part of the river 

 where the fall is the gentlest. The river has never l)een able to clear out 

 its glacial filling, except at the point where it flills to meet the Delaware. 

 A section across the river at Bethlehem shows the fresh and solid rock 

 rising just north of the river, in the new Nisky cemetery, and fifty feet 

 above the water, and also rising just behind the iron works in South 

 Bethlehem to the same height. Here we have a deep gorge of pre- 

 glacial origin, and studies along South mountain show that this prob- 

 ably extends from Easton to the Schu3dkill, with equally steep walls, as 

 we find the wells over the gorge from 70 to 120 feet deep, while on either 

 side at no great distance the solid rock outcrops. A section from Trex- 

 lertown to Alburtis starts from the middle of the limestone valley at 

 370 feet, with the rock 20 feet deep, and in three miles we find the rock 

 at the surface at 420 feet. A short distance further and we are at the 

 edge of a deep linionite mine, and this depth of fill continues till we 

 cross the railroad, when the rock outcrops on the slopes of South moun- 



