INTERPRETATION OF THE POTOMAC DEPOSITS 205 



and seaward tilting, erosion afforded the materials of the Potomac group. 

 The fact that these consist very largely of redeposited Piedmont crystal- 

 lines, and to a less extent of Appalachian materials, is therefore what 

 might be expected, but the circumstance that no clearly denned trace 

 of redeposited Newark materials has been found in the Potomac deposits 

 of Maryland is at first thought somewhat surprising. From this we must 

 infer either that the Newark was not to any great extent exposed to Poto- 

 mac erosion, or that its materials were not sufficiently consolidated to 

 permit of transportation except in a so finely divided condition as to be 

 unrecognizable. It is quite certain that during maximum Potomac sub- 

 sidence a large body of Newark materials, especially beyond the limits 

 of Maryland, was beneath tidelevel, and therefore not exposed to sub- 

 aerial influences. Inasmuch as the Potomac beds themselves, particu- 

 larly the basal ones, have since that date undergone considerable indu- 

 ration, often without the agency of iron oxide, we may suppose that the 

 subaerial Newark sandstones of that date, if consolidated at all, were 

 considerably less resistant than, for example, during the early Pleisto- 

 cene, in the deposits of which the Newark materials are abundantly 

 represented. 



The basal deposits of the Potomac group, produced by the initial 

 tilting of the continental border and described as the Patuxent forma- 

 tion, indicate in their arkosic character their proximity to the ancient 

 continent, the rocks of which had suffered extensive disintegration. 

 These features, which are so pronounced where the deposits lie adjacent to 

 highly feldspathic rocks, largely disappear where these rocks are poorly 

 developed or where the deposits themselves were evidently laid down at 

 some distance from the old shoreline. Rapid deposition in shallow waters 

 is seen in the crossbedded character of the strata and their rapid change 

 in lithologic characters. The presence of clay pellets and balls in the 

 sands of this formation, suggesting at first thought the existence of sub- 

 jacent pre-Patuxent sedimentary clays, may represent local shallowing 

 of the seas with the destruction by wave action of lately deposited Pa- 

 tuxent clay beds and the incorporation of their rolled materials into the 

 later deposits of the same formation. 



That the seaward tilting was not continuous or persistent in the same 

 direction is evidenced by the varying character of the deposits and the 

 stratigraphic relations which the several formations sustain to each other. 



The close of the Patuxent epoch was marked by the elevation of its 

 deposits and the trenching of its surface by streams. This was succeeded 

 b}' a subsidence which was emphasized to the landward by the occupa- 

 tion of the ancient valleys by swamp deposits. The tough clays of the 

 Arundel formation, charged with lignitic accumulations, in which tree 



