402 J. BAERELL RECOG^']TIOX OF ANCIENT DELTA DEPOSITS 



reasons the deltas of the present time are in a peculiarly disordered state. 

 Without the aid given by a deductive analysis of the delta cycle they 

 give but little clue to the place held in the scheme of sedimentation by 

 deltas during the earlier geologic ages. 



Resulting overemphasis of estuarine conditions. — The impress of re- 

 cG-nt broad crustal disturbances on the present surface forms of the world 

 was not fully perceived until after the principle of baseleveling by fluvia- 

 tile erosion had become accepted. Then it was seen that though pene- 

 plains in various degrees of preservation existed in many regions, they 

 v^'ere out of adjustment with the present baselevels and were being de- 

 stroyed instead of perfected by the newly initiated cycle of erosion. This 

 fact, which was at first used as an argument against the competence of 

 streams to produce peneplains, was shown by Davis to be, on the contrary, 

 an evidence of recent profound crustal disturbances. Other evidences of 

 the present physiographic youth of the lands are seen in the existence 

 of basin lakes of non-glacial origin and in estuaries. It has become well 

 recognized that lakes are impermanent geological features, and that 

 fiuviatile deposition is a more normal mode of continental sedimentation 

 than is lacustrine. In the literature of paleontology and stratigraphy, 

 however, the estuarine idea still plays an imduly large part in the inter- 

 pretation of the past. 



An estuary, according to the dictionaries, is an enlargement of a river 

 cliannel near its mouth, in which the movement of the tides is very 

 prominent. The principal existing estuaries — such, for example, as the 

 Saint Lawrence and the Thames — are all the result of recent coastal 

 subsidence and the drownins: of the lower river vallevs. The same move- 

 ment that resulted in the Gulf of Saint Lawrence has produced Long 

 Island Sound, Delaware, and Chesapeake bays: bodies of water which 

 sliow various degrees of salinity. They all bear the common marks of 

 being old river valleys trenched within uplands, and represent a reversal 

 from erosion to deposition so recent that the sediment carried in by the 

 rivers has not been sufficient to fill in the ancient valley. Tidal action 

 tends to keep open larger channels and maintain a greater depth of 

 water than waves alone could do, but tides do not account for the origin 

 of the present estuaries and will not prevent infilling with sediment. On 

 the contrary, the tides are effective agents in the restriction of broad 

 estuaries by building up tidal marshes. In the most striking example, 

 the Bay of Fundy, Lyell speaks of the rapid growth of tidal marshes 

 from the sediment precipitated on their surfaces at flood tide. In what 

 were once smaller estuaries, as at the mouth of the Savannah River, with 

 a tidal range of 7 feet, the coast chart shows how the estuary has been 



