410 J. BARRELL — RECOGNITION OF ANCIENT DELTA DEPOSITS 



parent absence of marine, brackish water, or even lacustrine faunas from 

 1j early the whole group show deposition in the vicinity of plant growth 

 and the presumed absence of permanent water bodies. In the Arundel 

 formation gypsum has been found. The bones of dinosaurs, turtles, and 

 crocodiles are fairly abundant, and occasionally large stumps are discov- 

 ered standing in the position in which they grew. Ascending through 

 the formations, however, the suggestions of fluviatile and terrestrial 

 sedimentation weaken in the Earitan, where logs of lignitized conifers 

 exhibiting teredo borings have occasionally been found, and in the over- 

 lying Magothy, as previously noted, the first clearly marine fauna occurs. 

 The value of these stratigraphic characters as criteria of fluviatile depo- 

 sition is discussed in the second part of this article, but they are men- 

 tioned here to show that the suggestions derived from the strata as well 

 as from the larger relations are those of a fluviatile rather than an 

 estuarine origin. 



The review of these characters permits the application of tlie princi- 

 ples discussed under the delta cycle as an explanation which connects the 

 physiographic relations of the Appalachians and the Coastal Plain with 

 the sequence of the formations and the final passage into marine condi- 

 tions, A new interpretation may then be given as follows : At the begin- 

 ning of sedimentation broad interior uplift was more pronounced than 

 subsidence of the Coastal Plain. This resulted in a dominance of sedi- 

 mentation over marine fransgression and a maintenance of the coast line 

 beyond the present limits of observation. The rivers were spaced at 

 sufliciently short distances, so that the deltas were confluent as a flat 

 Piedmont Coastal Pl^in, probably with a highly irregular and shifting 

 shoreline; but the entire absence of marine incursions between the larger 

 delta units or across their upper surfaces and the recurrence of uncon- 

 formities suggest that wluxt is now exposed was the landward side of a 

 gi'eat alluvial plain and the shore was continually maintained farther 

 seaward. If it were possible to follow the evidence beneath the present 

 sea the unconformities might be found to pass into terrestrial beds of 

 equivalent time value and the intervening formations which make up tlie 

 Potomac group miglit be found to pass seaward into marine topset beds 

 between tlie terrestrial intercalations. If there were in reality an axis of 

 no subsidence on the southeast during the Mesozoic, where now is o])en 

 water, to serve as a barrier, the deposition took place in a basin rather 

 than an estuary and maintained the region as the alluvial plain of the 

 trunk river which flowed through it. But of such a barrier there is no 

 structural suggestion nor theoretic need. 



Puring Cretaceous time subsidence became more marked than tilting, 



