MARYLAND GEOLOGICAL SURVEY 133 



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 coun- 

 try, and still others have reached the advanced stage of swamps or 

 meadows in which various types of vegetation are flourishing (Plate XX). 

 In Virginia, in addition to the usual undergrowth which is found in wet 

 places, the cypress has taken up its abode in these bogs and has converted 

 some of them into cypress swamps. For great stretches along the shore 

 the advance of the sea is indicated by well-washed cliffs, while in other 

 places the waves are found devouring beds of clay which are situated im- 

 mediately in front of lagoon swamps and separated therefrom by nothing 

 but a low superficial beach. These clay beds invariably lie at and below 

 water-level, are very young in age and evidently pass directly under the 

 beach to connect with the lagoon clay beyond. This interpretation is 

 made the more certain by the presence of roots in the wave-swept clays 

 which but a short time before belonged to living plants identical with 

 those now flourishing behind the beach and point to a time not far dis- 

 tant when they also were a part of the lagoon swamp behind a beach and 

 situated a little farther seaward. At Chesapeake Beach, in Calvert 

 count}^ a ditch has been cut through one of these beaches which shows 

 a continuous deposit of clay from a lagoon swamp out under the beach to 

 the Bay beyond. The waves are thus caught in the act of eroding the 

 upper portion of the lagoon deposit. 



From a large body of data gained throughout a wide area, it is evident 

 that the erosion which occurred during the interval between the elevation 

 of the Talbot terrace and the present subsidence of the coast was suffi- 

 cient to permit streams to cut moderately deep valleys in the former. It 

 would then appear that as the region was gradually lowered again be- 

 neath the present ocean the upper portions of the stream channel in time 

 passed below wave base and whatever has collected in them since that 

 period will be preserved beneath the advancing sea as a more or less f ossil- 

 iferous clay lens apparently unconformable beneath beach debris. 



The barrier beaches which exist at intervals along the Atlantic coast 

 of New Jersey, Delaware, Maryland, Virginia, and southward show us 



