1913.] NATURAL SCIENCES OF PHILADELPHIA. 121 



through finer and finer sandstones to the ordinary Brunswick shales. 

 The same red sandy mud makes up the bulk of both rocks, the pebbles 

 having simply been dropped into it while still soft. Similar rela- 

 tions appear in four distinct areas, all of the same general type, 

 although the shape of outcrop is modified by diabase intrusions 

 and local variations in the dip of the usually practically horizontal 

 beds. 



That these conglomerates have been deposited chiefly under water 

 is shown by the stratification and assorting of the pebbles, rough 

 though it may be, and by the occurrence of interbedded thin lami- 

 nated shales, which show such features as ripple marks and rill marks. 

 That the water was fresh is indicated by the absence of marine 

 fossils, and perhaps by the red color of the mud. The source, mode 

 of transportation, and of deposition of the pebbles remain to be 

 considered. 



Three possibilities at once suggest themselves: we may be dealing 

 with either talus broken from cliffs by wave or frost action, alluvial 

 fans, or glacial moraines. 



The first view, that the pebbles are talus blocks, was accepted 

 by Dr. KiimmeP^ in the New Jersey area. 



This conclusion is, however, quite inapplicable in the present 

 localities, for it is very evident that the nature of the pebbles bears 

 in general no relation to the character of the rock against which 

 the conglomerate lies. Dr. Kiimmel had observed the same thing, 

 and explained it as due to faulting, but, as shown above, this does 

 not occur in the present region. Gneiss pebbles are found most 

 abundantly at Momoe, where the floor is limestone, and limestone 

 pebbles west of Coopersburg, where the underlying rock is gneiss. 

 Indeed, there is now no outcrop of Shawangunk quartzite, such as 

 forms the majority of the pebbles, within twenty miles (although, 

 of course, it may have extended somewhat farther south in Triassic 

 times). Further, the fact that the limestone :s less rounded than 

 the much harder quartzite is just the opposite of what would be ob- 

 served in talus heaps rounded by wave action, but can be explained 

 according to the principle that the degree of rounding increases 

 with the distance of transportation, for the limestone rocks from 

 which the pebbles may have been derived outcrop nearly everywhere 

 within a mile or two of the edge of the basin. 



The application of the criteria for the recognition of alluvial- 



" Ann. Rept. State Geologist of N. J., 1897, pp. 52-58. 



