448 N. H. DARTON — LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



COLUMBIA FORMATION. 



In regard to this formation I have at present but little of general interest 

 to add to the statements of McGee. I do not find the interfluvial phase so 

 widespread as was originally supposed, and do find that certain high level 

 gravels and terraces of the Washington region are of Appomattox age. 



The Columbia terraces border the coastal plain rivers from the fall line 

 region to their mouths, and extend over wide areas in the low regions adjoin- 

 ing Chesapeake bay. The formation lies on terrace planes cut in the various 

 subterranes near tide-level and ranging in position from a few feet above to 

 a moderate distance below. 



The altitude of the terraces decreases eastward, and, as shown by McGee, 

 gradually increases northward through Virginia and Maryland. This in- 

 crease of altitude northward is similar to that of the Appomattox formation, 

 but less in amount, and probably indicates that part of the deformation is 

 of post-Columbia age. 



Following Columbia deposition came increased emergence, slightly greater 

 northward, and cutting of the present river channels to depths considerably 

 below present tide-level. Then followed submergence, which buried the 

 great river channels and the eastern edge of the coastal plain under tide- 

 water and ushered in the present epoch. 



The Displacement. 



Course and Relations. — In studying the physiography of the head of Chesa- 

 peake bay McGee found evidence of the existence of a longitudinal displace- 

 ment, which has depressed the level of the coastal plain considerably below 

 that of the Piedmont region. The line of dislocation is marked by steep 

 slopes along the margin of the Piedmont region, and was traced for several 

 miles along the side of the head of the bay. It was suggested that this dis- 

 placement probably extended from the Hudson river region southward along 

 the border of the coastal plain, following down the Anacostia river east of 

 Washington, and finally merging into a flexure in the Acquia creek region. 



I have found, however, in studying the border zone between the Piedmont 

 and coastal plain regions, that there is a line of dislocation some miles west 

 of this course which is practically continuous from at least as far north as 

 Newark^ Delaware, to south of Fredericksburg, and has had a more com- 

 plicated history than was at first supposed. The details of this history are 

 not as yet fully worked out, and at this time it is possible to give only a brief 

 general account of the more prominent consequences of the displacement. 



The dislocation traverses the crystalline rocks and Newark, Potomac, 

 Pamunkey, Chesapeake (?), Appomattox and Columbia formations, but the 

 Potomac, Appomattox and adjoining crystallines are the formations in which 



