134 THE GEOLOGY OF PRINCE GEOKGE's COUJSTTY 



Bay to Potomac River, as well as in Virginia and probably still 

 farther south. The key to the solution of the relations existing be- 

 tween the surficial deposits of Maryland lies almost exclusively in 

 a correct correlation of these terraces. Much light may be thrown 

 on this problem by a careful study of the Recent terrace now forai- 

 ing along the shores of the i\.tlantic Ocean and Chesapeake Bay and 

 its tributaries. A discussion of this terrace is given below. 



After the close of the post-Lafayette erosion period the Coastal 

 Plain was gradually lowered and the Sunderland sea advanced over 

 the sinking region. The waves of this sea cut a scarp against the 

 existing headlands of Lafayette and older rocks. This scarp was 

 prominent in some places and obscure in others, but may be readily 

 recognized in certain localities. As fast as the waves supplied the 

 material, the shore and bottom currents swept it out to deeper water 

 and deposited it so that the basal member of the Sunderland forma- 

 tion, a mixture of clay, sand, and gravel, represents the work of 

 shore currents along the advancing margin of the Sunderland sea; 

 whereas the upj)er member, consisting of clay and loam, was depos- 

 ited by quieter currents in deeper water after the shore line had 

 advanced some distance westward and only the finer material found 

 its way very far out. Ice-borne boulders are also scattered through 

 the formation at all horizons. 



After the deposition of the Sunderland formation, the country was 

 again elevated above ocean level and erosion began to tear away the 

 Sunderland terrace. This elevation, however, was not of long dura- 

 tion and the country eventually sank below the waves again. At this 

 time the Wicomico sea repeated the work which had been done by 

 the Sunderland sea except that it deposited its materials at a lower 

 level and cut its scarj) in the Sunderland formation. At this time 

 also there was a contribution of ice-borne boulders which were depos- 

 ited promiscuously over the bottom of the Wicomico sea. These are 

 now found at many places embedded in the finer material of the 

 Wicomico formation. 



At the close of Wicomico time the country was again elevated and 

 eroded, and then lowered to receive the deposits of the Talbot sea. 

 The geologic activities of Talbot time were a repetition of those car- 



