294 E. H. WILLIAMS, JR. EXTRAMORAINIC DRIFT. 



hem hill to the river. As the Lehigh is a stream of ^reat volume and 

 velocity, and since the filling remains as it was deposited, the postglacial 

 interval has been short. 



THE UNDERLYING ROCKS. 



In a few instances the surfaces are the remains of preglacial decom- 

 posed country rock left by the ice, as before noted, but the great majority 

 of exposures present glaciated and fresh country rock. The freshness may 

 be attributed to a denudation that has kept pace with decomposition, 

 and this might be plausible were the rock homogeneous and there ex- 

 isted no remnants of decomposed material for comparison. With ordi- 

 nary rocks there is little difficulty, even in the absence of striation, and 

 striation is uniformly absent over the region, in discriminating between 

 a glaciated surface and one etched by subaerial denudation. The latter 

 force would not work equally, however, under the varying caps of gravel, 

 sand and clay or all combined, and the equality of erosion points to 

 glaciation, while the freshness of the rock speaks of recency, as well as 

 does the comparison of the fresh and rotten forms of the same rock on 

 either side of the West Bethlehem ridge. There are thousands of these 

 fresh exposures, capped and uncapped, and they, are of gneiss, limestone, 

 sandstone and slate, so that the vaying rapidity of decomposition of the 

 rocks can be adduced as an argument for recency also, as all are equally 

 fresh. 



FREGLA CIA L SURF A CES 



A statement has been previously quoted that the Aquanchicola has 

 cut a new bottom through the solid Clinton shales since glacial times, 

 and another quotation stated that the Lehigh had made extensive cut- 

 tings in the same interval. Both are inaccurate. A study of the first 

 locality shows rock across what was called the preglacial channel and 

 above the present level of the river. The rock cut that is thought to be 

 the new channel is several hundred feet long. On the rock across the 

 " old channel " lies a thick bed of stratified sands capped by an equally 

 thick bed of Packer clay. In the first place, we are asked to believe that 

 the stream found it easier to cut through solid rock than to wash away 

 an equal amount of light sands and lighter clay ; in the second, we find 

 that the deposit of sands and clay never was as high as the supposed 

 barrier of solid rock, for the till has been traced to the tops of the two 

 sides of the creek, and also the capping of clay almost to the 500-foot 

 line. Both till and clay are found on the sides of the '* new rock chan- 

 nel," and in the case of the till from within five feet of the water's edge to 

 the top of the cut. This shows that the present channel of the Aquanchi- 

 cola is preglacial, and that the stream probably always flowed in that 



