GEOLOGY OF ILLINOIS. 



turbing influences to which the older strata have been subjected. The great 

 fault, which crosses this country below Cap au Gres, is the most remarkable 

 disturbance of the stratified rocks to be found within the limits of the 

 State, and to this disturbance is due the wide range of geological formations 

 that appear within the area of this county, comprising the whole range of 

 paleozoic strata, from the St. Peters Sandstone of the Calciferous period to the 

 Coal Measures, and including something over a hundred feet in thickness of 

 the latter group, a wider stratigraphical range of formations than is found in 

 any other county in this portion of the State. This fault intersects the Mis- 

 sissippi bluffs immediately below the high cliff of St. Peters Sandstone, to which 

 the name " Cap au Gres" or Sandstone Headland, was given by the French 

 Voyageurs, and- with a trend of east 10 south, it intersects the bluffs of the 

 Illinois about two miles below Monterey, crosses to the bluffs on the eastern 

 side of that stream, about five miles above its mouth, and after intersecting an 

 elbow of the river bluff in Jersey county for three or four miles, it is finally 

 lost in the valley of the Mississippi. The strata have not only been dislocated 

 by this disturbance, but there has also been a down-throw of the beds, to the 

 extent of at least seven or eight hundred feet on the western side of the fault, 

 while on the east, they remain but slightly elevated above their original hori- 

 zontal position, and are se'en dipping gently to the northeastward. 



To the northward of this axis, the Burlington limestone forms the bed rock 

 at the summit level of the dividing ridge between the two rivers, up to, and be- 

 yond the Pike county line, while to the southward, nearly all the highlands are 

 directly underlaid by the St. Louis limestone or the Coal Measures. The exact 

 line of this fault is hidden in the valley of a small stream, which enters the 

 Mississippi just below the Cap au Gres bluff, but immediately below this valley 

 the Lower Carboniferous limestones are seen standing in a nearly vertical 

 position, dipping south 10 west. 



The following wood cut will serve to illustrate the relative position of the 

 strata at this point, showing the Cap au Gres bluff of Lower Silurian strata on 

 the left, and the upturned edges of the Lower Carboniferous limestones imme- 

 diately below it on the right : 



o. Trenton Limestone. 

 6. St. Peters Sandstone. 



e. Burlington Limestone. 

 dt. Keokuk " 



e. St. Louis Limestone. 



