MARYLAND GEOLOGICAL SURVEY 77 



accumulations along the margins while the less active streams have left 

 the reentrant valleys practically unmodified. In southern Maryland the 

 streams which drain into Chesapeake Bay from the eastern slope of 

 Calvert county, as well as those which drain into the Patuxent river from 

 St. Mary's and Prince George's counties, have shorter courses than' those 

 which drain into the Patuxent from Calvert county or into the Potomac 

 from Prince George's, Charles, and St. Mary's counties. A similar con- 

 trast is obvious between the streams which enter the Atlantic ocean from 

 the Eastern Shore and those which enter Chesapeake Bay from the 

 same region. 



The cause of this shortening of streams on the northeast side of these 

 divides is probably due in part to a tilting toward the southeast which is 

 discussed later in this monograph, but also in a great measure, par- 

 ticularly along the Bay shore, to rapid wave erosion. The streams 

 draining the eastern slope of Calvert county and the northeastern slope 

 of St. Mary's and Prince George's counties were at one time longer, but 

 the recession of the shore line has shortened their courses by the cutting 

 away of their lower valleys. This is extremely well seen along the 

 Calvert Cliffs where the waves have advanced so rapidly on the land that 

 the former heads of stream valleys are now left as unoccupied depres- 

 sions along the upper edge of the cliffs, while other streams cascade from 

 the top of the precipice to the shore beneath, and still others more active 

 have been able to sink their valleys to the water's edge by a very sharp 

 decline (Plate XXI, Fig. 1). Other investigations have suggested 

 that rotation may have had some influence in bringing the streams 

 mentioned above into their present position, and although the streams 

 are short, it is probable that they have been somewhat affected by this 

 influence. 



Description oe Formations. 



The surficial deposits of the Atlantic slope consist of clay, loam, peat, 

 sand, gravel, and ice-borne boulders. South of Fredericksburg, Virginia, 

 they have not been mapped in detail, but from the Potomac valley north- 



