AGE OF THE TILL. 293 



The till has been traced to the Schuylkill at Ontelamiee station, on the 

 Pennsylvania railroad. There is a clay in the northern part of Reading 

 that may be bowlder-clay (subglacial) or due to ice dams, or perhaps to 

 decomposition of the local limestone. There are a few evidences that 

 the ice swung round the end of South mountain, but they are not 

 conclusive. The traces of ice-invasion become dubious over the western 

 end of the slate belt until we reach Jacksonville, where the thick masses 

 of gravel and bowlders are again found. The presence of the ice at this 

 point and at the Schuylkill shows that it must have covered the region 

 l)etween them. The next point to be studied is the extent of the inva- 

 sion beyond the Schuylkill and the point where its edge crossed the Blue 

 ridge. It is evident that ephemeral lakes were formed by the damming 

 of Lizard and of Mahoning creek, and two moraines have been found 

 crossing the valley of the former. In the eastern border it seems that 

 the ice did not cover either the Hexenkopf or Bowers rock, as it had 

 wasted its power in surmounting the ridges north of them. These two 

 peaks divided it into an eastern lobe which crossed the Durham valley ; 

 a middle one which crossed the Saucon vallev as far as Center Valley, 

 but did not go west of a line between that place and Friedensville ; and 

 a third which crossed the range l)ack of South Bethlehem (850 to 920 

 feet), but was turned westward by Bowers rock (1,020 feet) and went as 

 far as the Schuylkill. The southern border on the Trias has not been 

 fully determined, owing to the similarity of some of the till to the 

 rotten basal conglomerate of the Trias, l)ut it is hoped that the trap 

 pebl)les and glaciated outcrops may settle the matter in the absence of 

 fossiliferous foreigners. 



The age of the till has been briefly discussed in the American Journal 

 of Science for January, 1894, so that only a summary is necessary here. 

 We find : 



First, that the comparison of fresh glaciated surfaces with the remains 

 of preglacial decomposition shows that the former were recently eroded. 



Second, that the Packer deposits^ represent mainly preglacially oxi- 

 dized material, and their burden is generally fresh. 



Third, that the freshness of surfaces and burden must be balanced 

 against the bulk of the deposits, for if the freshness indicates recency, 

 the l)ulk stands for rai)idity. 



Fourth, that when the ice-front freed the mouth of the Lehigh it was 

 almost in the position where it formed the great moraine. This latter 

 may, therefore, be taken as a continuation of the work of glaciation in 

 the region, and we can apply the same rule of recency and rapidity to it. 



Fifth, that the scouring out of a filled gorge of the Lehigh is disproved 

 l)y finding the till and clay running uniformly from the top of Bethle- 



