132 THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



different manner (Plate XVIII). The lower portion carrying the marine 

 organisms points to salt-water conditions and contains remains of sea 

 animals which live to-day along the Atlantic coast. At the time when 

 this deposit was formed, then, the ocean waters had free access to the 

 region and the blue mud in which they are now imbedded and in which 

 they lived was a quiet-water deposit laid down some distance from the 

 land. Later, however, it would appear that a barrier beach was con- 

 structed shutting off a portion of the sea bed which had formerly been 

 occupied by marine animals and gradually allowing it to be transformed 

 from salt-water conditions to those of brackish water. In this brackish- 

 water lagoon the fauna changed to that found along our estuaries to-day 

 and huge oysters flourished and left behind them a deposit of shell rock. 

 With the bar advancing landward this lagoon was gradually filled up with 

 sand and gravel and finally obliterated. 



The upj)er unconformity, then, in the case of the fresh-water and the 

 brackish-water lagoons is real only in the sense that an unconformity in 

 a cross-bedded wave and delta deposit is real. There is, it is true, a lack 

 of harmony in the position of the beds and a sharp break is indicated, 

 but there is no indication of an appreciable time lapse between the clay 

 and the oyster bed on the one hand and the overlying sands and gravel 

 on the other, and the sea which eroded the clay to a fixed level imme- 

 diately afterward overspread the surface of the same with a veneer of 

 beach sand. There is, therefore, no time break indicated by this uncon- 

 formity and the lenses of swamp clay, as well as those carrying marine 

 and brackish-water organisms, are to be looked upon not as records of 

 elevation and subaerial erosion, but as entombed lagoon deposits made in 

 an advancing sea and contemporaneous with the other portions of the 

 formation in whose body they are found. 



The hypothesis here advanced is based on and reinforced by many ob- 

 servations along the present shores of the Atlantic ocean, the Chesapeake 

 Bay, and its estuaries. Each step in the process described is here illus- 

 trated and some of them are met with again and again. 



As one passes along the shores of Chesapeake Bay and of the rivers 

 which flow into it, stream channels are continuallv met which have 



