224 illinois state academy of science 



Folding 

 trenton time 

 The finding of apparently typical Kimmswick limestone 

 in Clark County in the supposed area of the Galena sea, 

 will introduce a question as to the amount of folding that 

 took place during or prior to this time along the zone of the 

 present LaSalle anticline which was considered the barrier 

 between the Galena and the Kimmswick seas. 



MAQUOKETA TIME 



The fact that three members of the Maquoketa, alter- 

 nately shale, limestone, and shale, were deposited over 

 most of this area and occur today at elevations varying over 

 3,000 feet, with only a total maximum change of 50 feet in 

 thickness, shows that the formation must have been de- 

 posited over the whole area under very similar conditions 

 relative to the source of supply, and on a practically flat 

 surface. 



If folding occurred prior to or during Trenton time ero- 

 sion probably removed all traces of any "Trenton relief" 

 before the Maquoketa was deposited in the zone of the pre- 

 sent La Salle anticline. Otherwise the Maquoketa would 

 not be as consistent in thickness and character as it is. 



SILURIAN AND DEVONIAN TIME 



These two geological ages, represented by a total thick- 

 ness of approximately 1,100 feet, must have seen tremend- 

 ous time intervals of erosion and deposition. Probably 

 earth movements and possibly folding took place. How- 

 ever, whatever the conditions may have been, they were 

 apparently similar over this whole area. The fact that, in 

 Parker Township, Clark County, the fold in the "Trenton" 

 has been shown to be parallel with the top of the Devonian 

 formation and similar to the fold in the Mississippian for- 

 mations suggests that folding in Silurian or Devonian times 

 in the anticlinal zone was very improbable. Undoubtedly 

 the thickness of individual Silurian and Devonian members 

 may vary over this area. 



As there are no signs from drill records that any con- 

 siderable erosion into the upper bed of the so-called Maquo- 

 keta has taken place, continued submergence probably fol- 



