CUMULOSE DEPOSITS 



301 



periods, "but which have remained in a condition of slight con- 

 solidation, and through subsequent elevation and weathering 

 form the soils. Such vary widely and abruptly. In the region 

 northeast of Washington, the Potomac formation consists of 

 feldspathic sands, gravels, and clays irregularly bedded and 

 often enclosing notable accumulations of rounded pebbles of 

 quartzite brought from the Appalachian and Piedmont regions. 

 The Appomattox formation, from which was derived surface 

 soil in the vicinity of the Kappahannock and Appomattox in 

 Virginia, is a yellowish or orange-colored clay and sand with 

 sometimes interbedded gravel. The Columbia formation which 

 yields the surface soil of the main portion of Washington City 

 and the immediate valley of the Potomac and tributary streams 

 southward, is a delta and littoral deposit made up of materials 

 worked over from the older Potomac and Lafayette formations 

 and also of granitic sands and clays from the decomposed rocks 

 of the Piedmont plateau. 



The clays of the Potomac formation above mentioned are at 

 times sufficiently homogeneous and plastic to be utilized in the 

 manufacture of brick, tiles, and pottery. The following table 

 shows the finely comminuted condition of the materials which go 

 to make up these clays in Maryland, as determined by Whitney.^ 



Mechanical Analyses of Potomac Clays, Maryland 





DXAMETKK 

 MM. 



Conventional Names 



Red Clay, 

 Tile 



Rkd Clay, 

 Puddling 



Blue Clay, 

 Stonkwakb 



2-1 



1^.6 



.5--.26 

 .25-.1 



.1-.05 

 .05-.01 

 .01-.006 



Fine gravel 



Coarse sand 



Medium sand 



Pine sand . 



Very fine sand 



Silt 



Fine silt 



0.00% 



0.00 



0.50 



2.63 



9.62 

 26.13 

 13.44 

 42.34 



0.31 % 

 0.82 

 2.69 

 . 3.23 



8.89 

 26.17 

 11.18 

 42.36 



0.00% 



0.00 



0.29 



1.27 



8.93 

 20.16 

 16.72 



.005-.0001 



Clay 



Total 



Organic matter, water loss . . 



50.02 





93.66% 

 6.34 



95.65% 

 4.35 



97.39% 

 2.61 



(2) Cumulose Deposits. — To be classed with the sedentary 

 deposits, in that they result from the gradual accumulation of 



^Bnll. 4, U. S. Dept. of Agriculture, 1892. 



