STRADIGRAPAVOOR THE POLOMAC GROUP 495 
the physical conditions which prevailed during the time immedi- 
ately preceding the opening of Potomac deposition. The results 
of recent work in the study of the physiography of the North 
Fic. 6. Section below Round Bay, Anne Arundel County, showing Raritan 
formation overlain by Matawan formation. 
American continent during the Mesozoic show that the eastern 
side of the country had been largely base-leveled, so that a long 
period of rock disintegration preceded the Potomac. With the 
advent of this period the land was tilted southeastward, and the 
increasing erosion brought about by this movement afforded 
the deposits of the Potomac group. That this movement was 
not continuous or persistent in the same direction is evidenced 
by the varying character of the deposits which have been already 
described. With the close of Potomac sedimentation a gradual 
transgression of the later marine formations across the margin 
