58 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVIXCE. 



peat filled with roots and tree trunks, lying on 8 feet of clear peat whicfi 

 merged with the overlying beds, and this in turn was underlain liy fossil- 

 iferous sand of late Neocene age. The thickness of the swamp deposits 

 decreases toward the periphery of the present swamp area, hut so few 

 excavations have been made along the border zone that the conditions of 

 thinning are not known. The upper beds of peaty materials merge gradu- 

 ally into the sands of the adjoining area, so that no boundary line can be 

 given. 



"The basin of the Dismal Swamp owes its origin to an extensive depres- 

 sion in the surface of the Columbia formation. At first this hollow was 

 probably a slough in the terrace surface. When the Columbia formation 

 was deposited James Eiver had essentially its present course, but emptied 

 into open water some distance northwest of the swamp. Its main current 

 appears to have built a bar or broad delta which extended eastward and 

 thus built up the terrace plain that lies east and southeast of Norfolk. 

 Between this delta and the steep slope at the edge of the highlands which 

 lie a short distance west of the Norfolk quadrangle there remained an area 

 of lowland, a slough which was not built up appreciably by the Columbia 

 deposits. When the delta was uplifted it became a high terrace with good 

 drainage conditions, while the slough became a swamp filled with luxuriant 

 vegetation, and it has so continued ever since. At first, when the vegetation 

 was young, relatively fine-grained peat accumulated, l)ut as the forest grew 

 older, roots and trunks were intermixed with the finer materials, and finally 

 the depression was filled up to the general level of the fountry by those 

 accumulations. It is now so remote from the larger drainage wt},ys and so 

 choked with canebrakes that its drainage is still very imperfect and the 

 swamp conditions continue over nearly all the original basin area. Lake 

 Drummond is no doubt the remaining portion of an original center pond, 

 probably greatly encroached on by the forests and canebrakes. It is prob- 

 able also that during some periods the lake was dry for a short time. Its 

 bottom has been raised somewhat by vegetal accumulations, but probably 

 its water level has just about kept pace with the general rise of the swamp 

 surface." 



