PAPERS OF GENERAL INTEREST 27 



to the area between Ozarkia and Cincinnatia, and the head 

 of the bay, as determined by deep well records, probably 

 did not extend north of the present site of Decatur, Illinois. 

 The succession of events during Chester time in the basin 

 consisted of a rhythmic series of advances and withdrawals 

 of the waters, with a consequent shifting of the shore lines, 

 similar to those which had taken place during Iowa time. 

 The sediments which accumulated in the basin during this 

 time were very different from those of Iowa time when 

 there was little but limestone, for there are extensive sand- 

 stone formations, and miuch shale is associated with the 

 limestone formations of the series. Where the Chester sec- 

 tion is most complete, in Pope and Johnson Counties, Illi- 

 nois, there is an alternating succession of sandstone and 

 limestone-shale formations, there being eight of these pairs, 

 or sixteen recognized formations in all. Each one of these 

 pairs of formations must represent a separate advance and 

 retreat of the waters of the basin, with the consequent 

 shifting of the shore line of the embayment alternately to 

 the north and to the south. During several of the periods 

 of withdrawal of the waters, the entire area of the basin 

 north of the present Ohio river must have been emergent. 

 forming a part of the dry land surface stretching away to 

 the north, northwest, and northeast. The evidence of such 

 complete withdrawal is the relation of unconfoimiity which 

 exists at a number of horizons between a limestone forma- 

 tion of the series, and the next succeeding sandstone. 



In general the sandstones must have accumulated in 

 proximity to the shore lines. With the advancing sea the 

 waves were constantly working over the accumulations of 

 beach sands, great quantities of which were spread over 

 the shallower portion of the sea bottom. With the advanc- 

 ing shore line a given point would be situated successively 

 on dry land, on the beach, in shallow water, and finally in 

 deeper water at a distance from the shore beyond where 

 the sand was being deposited, where fine mud and cal- 

 careous sedim.ents were accumulating. With the withdrawal 

 of the waters of the basin the reverse process must have 

 taken place. Deeper waters would become progi-essively 

 shallower and the wave movements of the waters would 



