110 THE GEOLOGY OF ST. JIARy's COUXTY 



this deposit was formed, 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 constructed 

 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 upper 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 

 immediately afterwards overspread the surface of the same with a veneer 

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

 unconformity 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 

 observations along the present shores of the Atlantic Ocean, Chesapeake 

 Bay, and its estuaries. Each step in the process described above is there 

 illustrated 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 continually met which have arrived at 

 more or less advanced stages in the above-mentioned process. Some are 

 in part converted into lagoons, by bars built across their mouths, others 

 show partial filling by mud washed in from the surrounding country, 



