36 E. II. Williams, Jr. — Age of the extra-moraine, etc. 



been no change of color from oxidation. No matter what the 

 dip and strike may be, they come sharply to the surface and 

 the cap is usually unconformable, except where a steep slope 

 has produced a local creep. For examples see Preliminary 

 Reports, 2d Geol. Surv. Penna., DD, Plate I ; D3, vol. i, 

 Plates II and III. These show that the till and clay did not 

 oxidize after deposition ; but that the already oxidized deposit 

 was laid upon a freshly glaciated surface. It also shows that 

 the date of glaciation is so recent that the gneiss, limestone 

 and slate have had no time to decompose or oxidize. In other 

 words, the glaciation has been quite recent. There are thou- 

 sands of such outcrops, and they exist at all elevations between 

 220-650 A. T., and under caps that vary from loose gravels to 

 compact clays. 



Fourth. There seems to have been but a short interglacial 

 period — if any existed at all — as the surface clay in many cases 

 shades gradually into the till ; or lies in a highly oxidized state 

 upon a glaciated surface of absolutely fresh country rock. 



Fifth. There has been an unbroken continuity in the till from 

 the crest of the Blue Ridge, west of the Lehigh Gap, to the 

 South Mountain. It has been followed down to and under the 

 flood, plain deposits of the present river systems of the region. 

 It has been a favorite amusement with some to argue about the 

 length of the interglacial period from the standpoint of the 

 time necessary for the Lehigh to erode a channel from a so- 

 called bed over Bethlehem hill, at 340 A. T. to its present level 

 at 220 A. T. It might have saved a great deal of bother if it 

 had been ascertained that the Lehigh was flowing on a rock 

 bottom. It has been ascertained that a deep gorge runs along 

 the north side of the South Mountain (whether continuous or 

 not is not yet determined) . of extremely narrow section and 

 with a bottom of fresh limestone at depths varying from sixty 

 to over one hundred feet below the present surface, and this 

 has been traced from Easton to within seven miles of Reading, 

 or from the Delaware to the Schuylkill. The limestone on the 

 north side drops almost vertically down to the maximum 

 depth, and the south side rises at a sharp angle. The section 

 of the deposits tilling this depression is identical in succession 

 with those of the rest of the valley. In other words we have a 

 depression that is glacial or pre-glacial, and is filled with the 

 same succession of sands, clays, till, etc., that cover the rest of 

 the valley ; but the individual members of the deposit (except- 

 ing the top one) are from five to seven times as thick. The 

 post-glacial erosion of the Lehigh and tributaries is a myth. 



There is but one conclusion to be derived from the above. 

 Granting that all the glacial formations (infra- and extra-mo- 

 raine) represent the whole work of the ice age in North 



