LARGER RELATIONS OF DELTAS 403 



converted into salt marshes, drained by a network of tidal (channels. 

 Thus the present complexities of the coasts broken by bays and estuaries 

 is a phenomenon as abnormal and geologically as temporary as the 

 warped and elevated peneplains which are beginning to be dissected. 

 They can not be safely used as illustrations for the interpretation of 

 ancient deposits, many of which were made during stages of crustal quiet 

 and low-land relief. 



In most of the so-called estuarine deposits of earlier geologic periods 

 there is in reality no evidence of tides nor of deposition within sub- 

 merged valleys previously eroded by rivers. Where modified marine 

 faunas indicate brackish water conditions, the present knowledge of the 

 physiography of earlier periods would point more commonly to partially 

 isolated bodies of shallow water existing because of downwarps of the 

 land, similar to Hudson Bay and the Baltic Sea. In other cases, how- 

 ever, it is probable that depauperated marine faunas associated with 

 fresh-water material may represent that combination of lagoon and 

 fluviatile conditions which marks the flat outer portions of large deltas. 

 Such lakes, bays, and sounds are well developed on the margins of the 

 Mississippi delta. The shifting of river channels and the pouring in of 

 flood waters serve to change greatly and suddenly the salinity of such 

 restricted water bodies and must produce environments especiall}'^ vari- 

 able and trying to their inhabitants. 



It would seem from the discussion of the delta cycle and its relation 

 to diastrophism and sedimentation that the term "estuarine deposit" is 

 very poorly chosen, and as a term of broad application should be avoided. 

 It must be confessed, however, that there is no entirely satisfactory sub- 

 stitute, since the words lagoon, bay, sound, and sea are in the English 

 language without sharply defined meaning. The word bay properly used 

 is, however, the best oC these? terms. P)ays are smaller and more inclose<l 

 water bodies than seas. Shallow seas and bays are normal physiographic 

 features of those continental elements which lie near sealevel. Bay for- 

 mations may therefore be laid down in irregular invasions by the sea 

 over a land of no relief or in the partially inclosed water bodies Avhich 

 result from delta-building, an invasion by the land against the shallow 

 sea. 



Modern illustrations of ancient interior deltas. — During the Paleozoic 

 the rivers commonly flowed into wide and shallow interior seas. The 

 latter no longer exist save in such restricted examples as the Baltic and 

 Hudson Bay, and into these no large rivers pour abundant waste. But 

 the recently submerged margins of certain deltas, or other conditions 

 inaugurating broad shallow waters, do bring about locally and tern- 



