MARYLAND GEOLOGICAL SURVEY 119 



advancing beach. The transverse section shows what is left of the 

 lagoon deposits of mnd carrying truncated stumps of cypress and other 

 trees which happened to be buried deep enough to escape the destructive 

 powers of the breakers. The broken line indicates the outline of the clay 

 lens. Fig. 7 is a section through the same region made at right angles 

 to the one just described. At D the breakers are forcing forward the 

 beach upon the meadow. Just off from the beach the waves have swept 

 away the sand and are eroding on the lagoon mud which reached out to 

 them under the beach veneer. At C the waves have succeeded in cutting 

 down the lagoon deposit to wave-base and have left behind a thin veneer 

 of sand and gravel as 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-erosion 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 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 imconformity 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 



