GEOLOGICAL FORMATIONS. 



843 



Beneath these various surface deposits are extensive layers of sands, 

 clays, gravels, marls, and matter of organic (chiefly vegetable) origin, 

 which slope gradually to the southeast and which have an aggregate 

 thickness of more than 525 meters (1,750 feet). These in turn rest 

 upon the strata of crystalline rocks which are exposed farther west, 

 but in the Dismal Swamp region are everywhere deeply buried. 



The formations above the rock floor represent all geological periods 

 from Lower Cretaceous to Recent. The greater part of them were 

 deposited unconformably, the general level of the Coastal Plain having 

 suffered numerous oscillations (luring the time in which they were 

 laid down. The following table of the several formations, giving 

 their period, general character, and thickness wherever the last was 

 ascertainable, is quoted verbatim from Mr. Darton's forthcoming 

 paper: 



Period. 



Recent. 



Pleistocene . 

 Neocene 



Eocene 



Cretaceous 



Fornication. 



Alluvium, etc. (unconform- 

 ity). 



Columbia (unconformity) 



Lafayette (unconformity) 



Chesapeake (unconformity). . 



Pamunkey 



Marine deposits (unconform- 

 ity). 



Magothy (unconformity) ... 



Potomac (great unconform- 

 ity). 



Crystalline rocks 



Character. 



River mud, marsh, beach sand, 



dune sand, etc. 

 Sandy loams, sands, and clays . 

 Gravel, orange sands, loams ... 

 Fine sands, clays, and diatoma- 



ceous deposits. 

 Glauconitic sands, marls, and 



clays. 

 Clays and sands 



Sands? 



Sands and clays 



Granites, gneisses, etc 



Thickness in me- 

 ters (and feet). 



0-18 (60). 



3-9 (10-30). 

 7.5-12 (25-40). 

 10.5-169.5 (35- 



565). 

 9?-90 (30-300). 



0-150 (500). 



? 



60-288? (200-960). 



The formations occupying the surface in the Dismal Swamp region 

 are the Chesapeake, Lafayette, Columbia, and Recent. Marine fossils 

 are abundant in the Chesapeake formation, fossil shells being present 

 in some deposits in such quantities as to afford marls valuable for 

 agricultural purposes. The inorganic matter which constitutes the 

 bulk of all the formations consists of the detritus of rocks in the 

 Appalachian and Piedmont provinces l to the west, which was carried 

 seaward by streams during past ages, just as is happening to-day. 



The Columbia formation, a comparatively thin layer of sands and 

 sandy loams, forms the surface of almost every part of the Dismal 

 Swamp region where very recent deposits (dune and beach sand, marsh 

 silt, swamp peat) have not been laid down upon it. This sheet varies 

 in thickness from G to 10 meters (20 to 35 feet). The formations lying 

 beneath the Columbia are naturally exposed only in the valleys of the 

 larger rivers, especially the Elizabeth, the James (Hampton Roads), 

 and the Nansemond. A section through part of the Columbia deposits 

 is exposed on the outer ocean beach, where it forms a low bench of 

 clay, either above high tide or between the levels of ebb and flood 

 (fig. 55). The materials belonging to this formation, which compose 



1 See footnote, p. 3!31. 

 235!t2— No. (>— 01 3 



