the Middle Atlantic Slope. 377 



rapidly to the plateau-like summit of the Catawissa hills; and 

 patches of loam similar to that forming the surface north of 

 the river occasionally appear on the slope from 100 to 250 feet 

 above the channel, and the hill-tops, 500 feet and less above 

 the river, are dotted here and there with well rounded quartzite 

 pebbles and bowlders two feet or more in maximum dimensions ; 

 the isolated loam patches and scattered bowlders alike rep- 

 resenting residuary traces of a once continuous formation now 

 largely removed. 



The overwash gravels are stratigraphically continuous with 

 and graduate imperceptibly into the terminal moraine, and are 

 manifestly the product of a rapid glacier-born stream with 

 considerable declivity. The loam of the base-level plain, on 

 the other hand, is unquestionably a deposit of slack waters ; 

 but it contains a notable element of partly oxidized rock-flour 

 (like that found at the head of Chesapeake bay), evidently of 

 glacial origin. Moreover, the high-level bowlders at the base 

 of , or incorporated within, the loam are much larger than those 

 transported by the present river, and were. evidently distributed 

 by floating ice of greater thickness than that now formed in 

 the same region. Both members of the formation thus attest 

 contemporary climatal refrigeration. 



As already indicated the high-level loam of the Bloomsburg- 

 Berwick section is continuous — save where locally cut off by 

 mountain ranges rising above its altitude — down the river to 

 Harrisburg, and the residuary cobbles and bowlders occur at 

 intervals over the slopes and within the inter- montane valleys 

 from which the loam has disappeared ; while the newer over- 

 wash gravels attenuate, their altitude diminishes, the materials 

 become finer, the terraces merge and finally disappear, and the 

 entire deposit fails above river level and the inner gorge is 

 completeiy lost, about the confluence of the "Western Branch at 

 Northumberland. Traced up stream the loam and the residuary 

 bowlders of the Columbia formation persist to the terminal 

 moraine where the soft contours of the loam-mantled plain dis- 

 appear beneath the knobby-surfaced moraine, and both loam and 

 bowlders are incorporated in the moraine material ; while the 

 overwash gravels lining the outer valleys lose their distinct 

 terracing, the cobbles increase in size and become less and less 

 perfectly rounded, the materials become more and more hetero- 

 geneous until they too merge into the terminal moraine, and the 

 entire valley is finally filled with aqueo-glacial gravels to a 

 height of 250 or 275 feet above the present level of the river 

 and to an unknown depth below. 



While the gravel and cobble deposit is simply the overwash 

 from the terminal moraine, the loam and high level residuary 

 gravels evidently represent a distinct and far older formation : 



