30 ILLINOIS STATE ACADEMY OF SCIENCE 



there began to accumulate a great series of sediments which 

 were terrestrial in origin rather than marine. These sedi- 

 ments consist of extensive beds of cross-bedded sandstones 

 which are commonly coarser in texture than the Chester 

 sandstones, some beds of which include great numbers of 

 smoothly rounded, white, quartz pebbles which vary in size 

 from one-fourth of an inch to nearly an inch in diameter. 

 These pebble beds or conglomerates are highly character- 

 istic of the Pottsville formation, and are widely distributed 

 in the hills of the elevated country crossing Illinois south 

 of Carbondale. Associated with the Pottsville sandstones 

 and conglomerates there are important beds of shale and 

 more or less thinly bedded sandstones, and also locally some 

 coal beds. The fossil remains which have been preserved 

 in the Pottsville beds are land plants. More or less frag- 

 mentary trunks of the Carboniferous tree, Lepidodendron, 

 are present in many places in the sandstones, and the shales 

 in places contain abundant, well preserved plants, most of 

 which are ferns or fern-like forms. 



Pottsville formations quite similar to those in southern 

 Illinois are widely distributed in North America. They are 

 present in the sections as far away to the southeast as south- 

 eastern Tennessee, and northern Alabama, and to the south- 

 west they extend into northern Arkansas. The exact con- 

 ditions under which such formations could have accumu- 

 lated are not easy to visualize. The source of the materials 

 in these Pottsville beds, including the vast numbers of white 

 quartz pebbles, is still a mystery to the geologist, and the 

 manner in which they may have been spread so widely upon 

 the land surface is not clear. Doubtless the land was low 

 lying, and broadly meandering streams probably were an 

 agent in spreading the materials. There must also have 

 been estuaries, and broad, shallow basins occupied by waters 

 in which quantities of mud accumulated, in which were 

 buried in places the remains of some of the plants which 

 lived near at hand. 



There are a few records in southern Illinois of a thin 

 limestone in this Pottsville series containing marine fossils, 

 which bears evidence that once at least, marine conditions 

 spread into southern Illinois in Pottsville time, remaining 



