468 J. BARRELL ORIGIN OF THE MAUCH CHUNK SHALE 



delta surface which in regions farthest removed from the sources of sedi-' 

 ment was occasionally in a swampy condition. Finally moderate uplift or 

 change in river gradients occiirred, with the result that river building was 

 changed to river erosion, and the land surface is at this epoch, from the 

 evidence of erosion, known to have extended over much of the Mississippi 

 valley. 



Relations of the delta plain to regions of erosion. — ^The material which 

 built up the Mauch Chunk foreland plain consisted of clay and loamy 

 clay and some loamy sand, transformed now into shale, sandy shale, and 

 argillaceous sandstone. Near the top appear occasional beds of con- 

 glomerate transitional to the Pottsville. The character of the Mauch 

 Chunk is much alike from the northeastern to the southwestern limits of 

 the southern anthracite basin,- a distance of 80 miles, and changes but 

 slowly in passing northwest, away from the sources of supply. This 

 approach to uniformity of composition over a broad area is characteristic 

 of the delta plains of large river systems, where the material has already 

 traveled the greater part of its distance before reaching the upper limits 

 of the delta. It implies, therefore, the existence of a long intermediate 

 graded portion of the river course between the region of the headwaters 

 supplying the material and the region of its deposition. The rate of sup- 

 ply of the sediment was sufRcient to continually bar the sea from the 

 anthracite regions, notwithstanding the combined epeirogenic and geosyn- 

 clinal subsidences of Lower Mauch Chunk times. In this way was built 

 up a deposit reaching a maximum thickness of 3,000 feet and of which 

 the remaining areas constitute but a remnant of the original volume. The 

 fineness and uniformity of the material imply leisurely currents; and 

 that the accumulation was slow is indicated by the thickness of the lime- 

 stones laid down during the same interval in the Mississippian sea. That 

 the erosion of this material did not transform the topographic character 

 in the regions of erosion is suggested by the approximation to uniformity 

 in the material making up the whole formation, the greatest mass of pure 

 shales being found in the lower part of the formation. The volume of 

 the land above baselevel which supplied the material for the Manch Chunk 

 was therefore large in comparison with the volume of the formation, 

 unless, indeed, a slow rise pari passu of the regions of erosion be postu- 

 lated corresponding to the amount eroded. 



Character of the delta surface. — The surface of the Mauch Chunk delta 

 in all of eastern and central Pennsylvania seems to have possessed a fair 

 grade to the westward, sufficient to prevent the development of broad 

 swamp areas, since in those regions decolorized shales are practically 

 absent. The grade which this indicates 'varies with the climate, since in 



