MARYLAND GEOLOGICAL SURVEY 191 



the sinking land carries it below the reach of the waves. At B the 

 lagoon deposit was not thick enough to reach the zone of wave-ero- 

 sion and simply grades up into a thick deposit of sand and loam which 

 passes out toward A. 



The second category of clay lenses, namely those carrying marine 

 and brackish-water organisms are understood to have been formed 

 in a somewhat different manner. 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, the ocean waters had free 

 access to the region and the blue mud in which they are now imbed- 

 ded 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 uncon- 

 formity 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 contempo- 

 raneous with the other portions of the formation in whose body they 

 are found. 



