FLOODPLAIN ORIGIN 467 



a shallow sea progressively invaded Pennsylvania from the southwest 

 and reached as far north as central Pennsylvania, but it did not at any 

 time reach as far northeast as Pottsville. 



Following this main incursion came a period of land extension. Fer- 

 ruginous sands and muds were now deposited over almost the entire state 

 of Pennsylvania. Along Chestnut ridge, in southwestern Pennsylvania, 

 Eogers first noted the presence of two beds of coal imbedded in the shales 

 immediately under the Pottsville conglomerate.* The upper shales are 

 characterized in western Pennsylvania by the presence of iron ore also. 

 Of this ore Eogers states : 



"The chemical nature aud geological relations are very similar to those of 

 the carbonate of iron of the coal strata, and the conditions under which it 

 originated were obviously very nearly identical with those which produced 

 that variety. The Umbral (Mauch Chunk) shales contain, especially in their 

 southwest outcrops, as in Somerset and Fayette counties, a species of ore iden- 

 tical with the ordinary compact or earthy carbonate of the Coal Measures. 

 This latter kind belongs to a small subordinate group of coal-bearing rocks, 

 which, in the districts mentioned, underlie the true Serai (Pottsville) con- 

 glomerate, or constitute its lowest member, indicating a gradual transition 

 from the Umbral series. "f 



Fuller, in the Elkland-Tioga folio, dealing with a quadrangle on the 

 northern boundary of Pennsylvania, gives a section of the Mauch Chunk, 

 which is here only a little over 100 feet thick. In the upper portion is 

 noted the occurrence of 4 feet of pure bog iron ore. Many iron ores 

 are concentrated by underground waters, for which reason no mention 

 has been made of the ores at the very base of the Mauch Chunk shales 

 in Huntington county. But these upper ores, similar to those of the 

 coal strata and associated in some places with coal, in another locality 

 spoken of as a "pure bog iron ore," may be taken as strong evidence of 

 river-swamp conditions at the time of origin of the strata. 



As noted in the description of the relations of the Mauch Chunk to 

 the other formations, there is thought to be a considerable erosion inter- 

 val in western Pennsylvania separating the Mauch Chunk from the 

 Pottsville, representing the time of deposition of the Lower Pottsville 

 and possibly some of the Upper Mauch Chunk of eastern Pennsylvania. 



It would seem, therefore, that while in eastern Pennsylvania subsidence 

 and river building were continuous, in western Pennsylvania, from the 

 time of the formation of the Greenbrier limestone, the crust was practi- 

 cally stationary or slightly uplifted, with the result that the shallow sea 

 was filled with mud and finally changed by river outbuilding into a broad 



• Geology of Pennsylvania, vol. 11, part 1, 1858, p. 472, 

 t Ibid., vol. 11, part 11, p. 734. 



