MARYLAND GEOLOGICAL SUKVEY HT 



places about Chesapeake Bay local Pleistocene deposits contain g-reat 

 numbers of marine and estuarine mollusks. 



At almost every place where good sections of Pleistocene materials 

 are exposed the deposit from base to top seems to be a unit. At some 

 places, however, certain layers or beds are sharply separated from the 

 underlying beds by irregular lines of unconformity. Some of these 

 breaks disappear within short distances, showing clearly that they 

 are only local phenomena in the same formation, the result of con- 

 temporaneous erosion by shifting shallow-water currents. Whether 

 all these breaks would thus disappear if sufficient exposures occurred 

 to permit the determination of their true nature is not known. An 

 additional fact which indicates the conteriiporaneous erosive origin 

 of these unconformities is that in closely adjoining regions they 

 seem to have no relation to one another. Inasmuch as the Pleis- 

 tocene formations lie in a nearly horizontal plane it would be possi- 

 ble to connect these separation lines if they were subaerial uncon- 

 formities due to an interval of erosion. In the absence of any 

 definite evidence that these lines are stratigraphic breaks separating 

 two formations, they have been disregarded. Yet it is not improb- 

 able that in some places the waves of the advancing sea in Sunder- 

 land, Wicomico, and Talbot time did not entirely remove the beds 

 of each preceding period of deposition over the area covered by the 

 sea in its next transgression. Especially would materials laid down 

 in depressions be likely to persist as isolated remnants which later 

 were covered by the next mantle of Pleistocene deposits. If this is 

 the case each formation from the Lafayette to the Wicomico is prob- 

 ably represented by fragiuentary deposits beneath the later Pleis- 

 tocene formations. Thus in certain sections the lower portions may 

 represent an earlier period of deposition than that of the overlying 

 beds. In regions where pre-Quaternary materials are not exposed 

 at the bases of the escarpments each Pleistocene formation near its 

 inner margin probably rests upon the attenuated edge of the next 

 younger formation. Inasmuch as lithologic differences afford insuffi- 

 cient criteria for separating these late deposits, and as sections are 

 not numerous enough to furnish distinctions between local inter- 

 formational unconformities and widespread unconformities result- 



