406 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



conclusion as to the larger relations and the decision as to an estuarine, 

 delta, lacustrine, or other mode of origin should be tested further by 

 another set of criteria derived by comparing the broader relations of 

 sedimentation under those different conditions. The previous discussion 

 has pointed to the large place which delta formations should occupy 

 under conditions which have not uncommonly occurred through geologic 

 history, and it is to be concluded that in statements as to mode of origin 

 conditions of lacustrine and estuarine deposition have been too freely 

 invoked and without adequate proof. On the other hand, fluviatile depo- 

 sition on deltas or in basins has not received proper recognition, qualify- 

 ing this statement, however, with the corollary that the shallow pans of 

 more or less permanent water on floodplains give minor lacustrine phases 

 connected with fluviatile aggradation. 



The principle of the delta cycle may, accordingly, be used as one cri- 

 terion of the mode of origin, and, as an illustration, it will be applied to 

 the late-Mesozoic deposits of the Atlantic Coastal Plain. It will be seen 

 to bring forth different conclusions from those stated in the publications 

 regarding these formations. It is possible to discuss the problem of 

 origin only because of the excellent and authoritative stratigraphic work 

 done in recent years by Darton, Clark, Kiimmel, Shattuck, Miller, Bib- 

 bins, and others. Their field work is here wholly accepted and the dis- 

 cussion turns merely on the interpretation as to origin. 



Sedimentation began near the close of the Jurassic* as a result of a 

 crust tilting, which brought the old crystalline floor somewhat below 

 baselevel. Eepeated movements of partial uplift alternated with con- 

 tinued depression, so that the formations are separated by unconformities 

 which may represent longer intervals of time than the deposition of the 

 sediments. The conditions of sedimentation remained much alike, how- 

 ever, through late Jurassic and Lower Cretaceous time and resulted in 

 the formation of the Potomac group, which recent studies have separated 

 into four formations — the Patuxent, Arundel, Patapsco, and Earitan. 

 These and the succeeding deposits are well described in the Patuxent and 

 Philadelphia folios, so that a full description may be omitted from the 

 present paper. There is preserved throughout the group a diversified 

 land flora. Less abundant remains of a land fauna are present, but 

 no marine fauna. Above the Potomac group occurs the Magothy forma- 

 tion. It is characterized by the occurrence in places of a marine fauna, 

 as in the vicinity of Earitan Bay, New Jersey, but no marine fossils have 

 been observed to the southward, extending from Burlington County, 

 New Jersey, into Maryland. The Magothy contains also, however, a 



* By some geologists regarded as earliest Comanche. 



