28 ILLINOIS STATE ACADEMY OF SCIENCE 



again initiate the distribution of sands over regions which 

 had been limestone-shale depositing areas. These newer 

 sand deposits, however, would soon become a part of the 

 dry land surface where they would be subject to erosion, and 

 a large portion of the material would be washed down again 

 into the sea to be reworked by the waves, and eventually the 

 final land surface would be constituted of the limestone 

 strata which had been deposited at some distance off shore. 

 With the next advance of the waters in the basin the cal- 

 careous land surface would become the floor upon which the 

 next succeeding sandstone formation was deposited uncon- 

 formably. 



It is not clear that every one of the Chester sandstone 

 formations which have been recognized in southern Illinois 

 exhibits a condition of unconformity along its belt of out- 

 crop with the underlying limestone, but the presence of un- 

 conformity or the lack of such a relation at these horizons 

 is indicative of the position of the fluctuating Chester shore 

 lines relative to the present outcropping belt of the for- 

 mations. In the greater withdrawals of the waters of the 

 basin the whole of southern Illinois doubtless became a part 

 of the dry land surface, but in the lesser withdrawals the ex- 

 treme southern position of the shore line was north of the 

 present position of the Ohio river. During the entire suc- 

 cession of events of Chester time, the shifting shore lines 

 must have repeatedly occupied every part of southern Illi- 

 nois. 



The original source of the sand which was finally con- 

 solidated in the sandstone formations of the Chester series 

 may have been far away. Rivers draining the country far 

 to the north must have emptied into the Illinois embayment 

 during this time, and the sand and silt transported by them, 

 perhaps from as far away as the Canadian highlands, may 

 have been the original source of much of the material. The 

 Ozark land to the west was probably a low lying region dur- 

 ing much of the time, with sluggish streams which did not 

 bring much land detritus into the basin, and there is no evi- 

 dence that Cincinnatia was ever at any great elevation above 

 the sea. The fact that several of the Chester sandstones 

 become much reduced in thickness to the west is perhaps 



