524 



THE COMANCHEAN PERIOD 



The clay, sand, and gravel are disposed irregularly, doubtless 

 the result of the physical conditions where the sedimentation took 

 place, conditions which might have existed along the lower courses 

 of rivers or at their debouchures, where shore-waters had little 

 effect upon them. 



In addition to the clastic sediment, there is a little lignite, and 

 some iron ore, and though both are widely distributed, neither 

 is of much commercial value. 



Structure and thickness. The Potomac and Tuscaloosa series 

 are nearly horizontal, with a gentle dip seaward. The Potomac 



Fig. 446. Section showing relations of various members of the Coastal series. 

 C, Comanchean; K, Cretaceous; E, Eocene; M, Miocene; PI, Pliocene; Q, Quater- 

 nary. 



series rests unconformably on Triassic and other formations (Fig. 

 446), and the Tuscaloosa on Paleozoic or older strata. Both series 

 are overlain unconformably by the Upper Cretaceous. The Poto- 

 mac formations reach a thickness of 700 feet in but few places. 

 The thickness of the Tuscaloosa series is about twice as great. 



Western Gulf Region. The system is more fully represented in 

 Texas than farther east, but its stratigraphic relations are the same. 

 The beds appear at the surface over an area distant from the coast, 

 dip seaward at a low angle, and are concealed near the coast by 

 younger formations. The lower part of the system (the Trinity 

 series) is perhaps the time equivalent of the Potomac, while 

 the uppermost series (the Wichita) is probably younger than any 

 part of the system on the Atlantic coast. Some parts of the system, 

 especially the middle (Fredericksburg) are marine, and some ter- 

 restrial. The marine part includes much limestone. The system 

 here is much thicker than farther east, ranging from 1,000 feet to 

 about 4,000. 



From Texas, the Comanchean formations, or some of them, 

 originally spread northward into Kansas, northwestward to Colo- 

 rado, and westward to Arizona. Though they appear at the surface 

 in small areas only, their extent may be considerable beneath younger 

 formations. 



