FORMATIONS AND PHYSICAL HISTORY 523 



FORMATIONS AND PHYSICAL HISTORY 



Atlantic and Gulf border regions. That part of the Coman- 

 du-an system along the Atlantic coast is called the Potomac series; 

 the part along the eastern Gulf coast, where conditions of sedi- 

 mentation appear to have been similar, is the Tuscaloosa series. 

 Fig. 445 shows that the system outcrops near the inland margin of 

 the Coastal Plain. It is the lowest of the Coastal Plain formations. 

 Neither the Potomac nor the Tuscaloosa series is believed to repre- 

 sent the whole of the period, and the two are not strictly contem- 

 poraneous. 



Conditions of origin, and constitution. By the beginning of 

 the Comanchean period, both the Appalachian Mountains and the 

 area to the east had been degraded well toward base-level, so that 

 little warping of the surface appears to have been needed to convert 

 portions of the coastal lands into sites of deposition, though more 

 may have been necessary to provide lands high enough to furnish 

 abundant sediments. The peneplanation of the eastern mountains 

 during the Jurassic period was no doubt attended by deep decay of 

 the underlying rocks, and the consequent accumulation of a heavy 

 mantle of residuary earth. The warping which inaugurated the 

 Comanchean period seems to have involved a rise of the Appalachian 

 tract, and a consequent rejuvenation of the drainage from it, while 

 the coastward tract was left relatively low and became a zone of 

 lodgment for the sediments brought down by the quickened drain- 

 age from the west. Lakes, marshes, etc., probably were features 

 of the lodgment area. The deposits consist of gravel (or conglom- 

 erate), sand (or sandstone), and clay, largely uncemented. 



The gravel and sand came chiefly from formations to the west. 

 Both are arkose (containing particles of crystalline rock, not de- 

 cayed when deposited) locally, showing that erosion sometimes 

 exceeded rock decay. This suggests high land to the west whence 

 the sediments were derived, and is one of the reasons for the belief 

 that it was tilted upward at this time. Beds of clay in the Potomac 

 series have been utilized extensively, especially in New Jersey, 1 for 

 the manufacture of clay wares. Some of it is notable for its bright 

 and variegated colors, black, white, yellow, purple, and red being not 

 uncommon. White is to be looked upon as the normal color; the 

 others are the result of various impurities, the black being due to 

 organic matter. 



1 Cook, Geol. Surv. cf New Jersey, 1868, and Kiimmel, 1904. 



