124 PROCEEDINGS OF THE ACADEMY OF [March^ 



rounded pebbles and boulders of various Paleozoic rocks, chiefly 

 quartzites, derived from the mountains to the north, were deposited 

 in a rudely stratified sheet over the valley floor in sufficient quantity 

 to almost completely cover the underlying limestone rock.^* 



At the south side of the valley such drift extends just up to the 

 Triassic contact, and many of the drift boulders are essentially 

 similar in size, shape, and degree of rounding to those weathered 

 from the Triassic conglomerate, and in fact can only be distinguished 

 by the color, which is yellow or brown instead of red. 



If there were in Triassic times glaciers in the mountains, they 

 must have formed terminal moraines, and drift from these would 

 naturally be occasionally transported on cakes of ice do'v\Ti the 

 streams issuing from the glaciers, and be dropped whenever this 

 ice melted. Some could not fail to be carried out into the shallow 

 lakes or ponds in which the Triassic red muds were being deposited, 

 and would then be dropped into this mud without disturbing its 

 stratification, as we actually find to be the case. Again, as the 

 distance out from the margin of the basin to which the pebbles 

 would be carried have nothing to do with their size, but only with 

 the size of the ice blocks and the resulting rate of melting, they 

 should simply become fewer in numbers, rather than less in size, 

 toward the center of the basin. As noted above, this is exactly 

 what does occur. 



It is not intended to imply that all of the materials of these 

 conglomerates had this origin. The smaller, subangular limestone 

 and gneiss pebbles and the green shale-flakes were, no doubt, car- 

 ried largely by direct stream action. But the writer feels convinced 

 that the features shown by the great mass of quartzite boulders 

 constitute a good indication of the correctness of Fontaine's and 

 Russell's theory that glaciers existed in the Appalachian Moun- 

 tains during late Triassic times. 



Summary. 



The shape of outcrop and structure of the three subdivisions of 

 the Pennsylvania Triassic implies either a profound fault on the 

 north side of the basin or progressive overlap in that direction on 

 an extensive scale. The latter view is shown to be the most satis- 

 factory one in this region. The conglomerates developed along 



^^ Williams, E. H., Extra-morainic drift between the Delaware and the Schuyl- 

 kill, Bull. Geol. Soc. Amer., V, 281-296, 1894. 



