DELTA DEPOSITS 745 



mation and thin coals only at the very top. It is concluded that these sands 

 were derived from deltas, such as the one described by Mr. Branson, of 

 streams heading in the older rocks to the east and flowing across a coastal 

 plain to the inland sea, and there distribut(Hl nlonu: the shore by shore cur- 

 rents. 



Mr. Arthur Keith called attention to a statement by l*rofessor Grabau that 

 a certain type of cross-beddini; characterized continental deposits, cited the 

 eonmion occurrence of cross-bedding in marine and continental be<ls, and re- 

 quested Professor Grabau to inform the Society as to the difference seen be- 

 tween marine and continental cross-bedding. Such distinctions would be very 

 useful as criteria on account of the easy recognition of cross-bedding. 



Prof. E. T. Wherry : At the northern edge of the Triassic in eastern Penn- 

 sylvania conglomerates occur which, judging from their character and dis- 

 tribution, are to be regarded as delta deposits. 



Dr. H. B. KtJMMEL raised the question whether a distinction was not to be 

 made between ancient alluvial fans wholly subaerial in origin and formed 

 wholly independent of bodies of standing water and true deltas. Some of the 

 instances of deltas cite<l in the discussion seem rather to be alluvial fans 

 similar to those now being formed in the west at the foot of high mountains. 



The authors made the following replies : 



Professor Barrelt-: A study of criteria to separate the submarine and sub- 

 aerial deposits of deltas brings out clearly the need of more refined compara- 

 tive studies of modern sedimentation under a variety of conditions. It is 

 evident, however, that certain criteria are not to be used according to their 

 mere presence or absence, but rather according to their specific characters and 

 dominance. Cross-bedding may be cited' as an example, present in marine, 

 fluviatile, and eolian sands, but presumably showing distinctive characters in 

 each case. 



In reply to the comment of Dr. G. I. Adams as to the apparent confusion 

 which is being introduced of marine formations with delta deposits, it may be 

 said that by definition deltas involve both subaerial and subaqueous phases, 

 since they are built against permanent bodies of water. On the other hand, 

 marine beds of mud or sand should not be regarded as belonging to deltas un- 

 less there is evidence that they passed on the landward side into synchronous 

 fluviatile deposits. There is no inherent distinction between the Iwttomset 

 beds of a delta and the offshore bottom deposits opposite a river mouth, where 

 deltas have been unable to form. Delta conditions involve the conception of 

 certain relations between contemporaneous fluviatile and offshore deposits, 

 not the nature of the beds themselves, for these are merely fluviatile and 

 marine. 



Professor Grabau: The two types of delta — sea-shore and subaerial fan — 

 are probably to be recognized in the older deposits. Of the sea-shore deltas, 

 in part snl)aerial, the Bald Eagle, .Juniata, Tuscarora, and perhaps Shawan- 

 gunk and Pottsville. The Longwood is most probably a subaerial fan, as 

 there are no known marine parts of this deposit. The same is true of the 



