478 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



conglomerate has thinned considerably in Green Pond Mountain. The 

 Shawangunk conglomerate, its equivalent at the Delaware Water Gap, is, 

 according to the careful measurements of Mr. Billingsley, about 1,900 

 feet thick, the distance between the two points being less than 24 miles. 

 If the conglomerate were a marine formation, this thinning toward the 

 source of supply would suggest progressive overlap against the old land. 

 If, on the other hand, the conglomerate is of terrestrial origin, this thin- 

 ning may be regarded as due either to subsequent erosion of the upper 

 beds before the deposition of the succeeding formation or to a backward 

 overlap of the strata of a growing delta, in which deposition with decreas- 

 ing gradient will successively extend farther up the stream in the direc- 

 tion of the source of supply. 



The name "Longwood shales" was also introduced by Darton for the 

 red beds everywhere succeeding the Green Pond conglomerate, and has 

 been generally adopted for these beds, though local names are used in 

 New York (High Falls) and Pennsylvania (Bloomsburg). At the type 

 section in Longwood Valley east of Middleton, Xew Jersey, the forma- 

 tion is estimated to have a thickness exceeding 200 feet. In the northern 

 end of the fault-block the Longwood shale has decreased to 119 feet in 

 thickness, the beds being well exposed in the railroad at Cornwall. The 

 beds are coarser below, apparently grading down into the Green Pond 

 conglomerate and become finer upward. The shale is mostly of a bright 

 red color, but some thin layers of lighter colored shales are found inter- 

 bedded. The beds are terminated in the northern part of the section at 

 Cornwall station by a 1-foot bed of shaly, brecciated limestone, followed 

 by sandstone and light colored quartzites carrying the Upper Siluric 

 (Decker Ferry) fauna. 



The Longwood shales here as elsewhere show characteristics pointing 

 to a subaerial origin. This is seen not alone in their color, but also in 

 the presence of mud cracks and of clay galls in the more sandy layers. 



Although the greatest thickness of the beds in this ridge is only some- 

 thing over 200 feet, their thickness across the valley, in the Kittatinny 

 Mountain region, is over 2,300 feet. Whether the great reduction on the 

 east is largely due to original difference in deposition or mostly to subse- 

 quent erosion can not now be determined. Certainly there was extensive 

 erosion of these red beds in early Upper Siluric time ; for the beds suc- 

 ceeding the Longwood in the Green Pond-Skunnemunk outlier are of- 

 Upper Monroe age, beginning with the Decker Ferry formation, which 

 includes at the top the Cobleskill and is succeeded regularly by the 

 Rondout and Manlius. The thicker series of Longwood beds across the 



