290 E. H. WILLIAMS, JK. EXTRAMORAINIC DRIFT. 



capped the region. No snbglacial till is shown. The bed of gravel lies 

 on the eastern end of the southern side of a ridge and where it falls rapidly 

 to the Manokisy. A section through the ridge shows that the underlying 

 rock has lost its decomposed cap on the northeast slope, which rises fifteen 

 to one, and up which the ice came, but retains it to a great depth on the 

 southern side (with a similar slope), down which the ice passed to the Le- 

 high, 130 feet below. From this it seems that the ice could not accom- 

 modate itself to the sudden angle in the ground, and an arch must have 

 been formed on its under side, with the usual crevasses, so that the press- 

 ure was not exerted till just before reaching the Lehigh. This stretch of 

 ice was from 150 to 280 feet beneath the lake surface, and the arch was used 

 by the lake waters in their escape toAvard the Delaware. The ground falls 

 rapidly on the northern bank of the Lehigh from Allentown to Easton, 

 and the existence of similar deposits in places along this line and just north 

 of Hellertown, where the waters from the Saucon valley would attempt to 

 escape (and along a similar ridge), lends strength to the theory of the 

 formation of these deposits by subglacial drainage. During the advance 

 of the ice there was no stability to the deposit, and it was continually 

 carried aAvay by the moving ice, as is shown by the finding of thick sub- 

 glacial till beneath the gravels and above the decomposed limestone ; 

 but this conclusively proves that the till acquired a great part of its river 

 gravels from this source. This gravel deposit is not a river bed, as there is 

 no continuous deposit across the Manokisy at Bethlehem, and it was 

 formed after the advent of the glacier and before the deposition of clay, as 

 it is enclosed between till and clay. It probably began to accumulate after 

 the motion of the ice had ceased and before it had melted over the spot. 

 The subglacial waters would increase the size of the arch under the ice, 

 and stratification went on proportionate to the dissolving of the ice. 

 When the ice was completely removed erosion began and continued till the 

 ice-foot had retreated for some distance down the ridge, and when slack 

 Waaler occurred and the clay began to deposit, the eastern end had been 

 cut off, and the clay rested unconformably on the eroded surface, as can 

 be seen in plate 10. It remains to note that when Mr Lewis * visited the 

 place there were no bowlders in sight in the gravels. Their occurrence 

 is not common, but one, of a spindle shape, is now exposed on the 

 western side of the pit and about fifteen feet from the top of the gravel. 



We have, therefore, a stratified gravel on the south side of the ridge 

 capped with clay, and a glaciated and fresh rock surface similarly capped 

 on the north side. 



The Till and its Age. — This exists as ground, lateral and terminal mo- 

 raines. The first varies from the ordinary formation by the persistent 



*See Report of Progress, Second Geol. Survey of Pennsylvania, v^^:^, vol. i, p. 48. 



