60 IOWA ACADEMY OF SCIENCES. 



course, are bevelled or planed off to conform with the present 

 surface of the region. 



The nearest exposures of rocks of the. same age are in the 

 Ozark region, and there also are the closest stratigraphical 

 equivalents. Other localities are no nearer than northeastern 

 Iowa. 



Winfield Limestone. — This formation is exposed only in the 

 immediate vicinity of the Cap-au-Gres fault. Its greatest 

 exposed thickness is half a mile north of Winfield, in Lincoln 

 county, Missouri, where it rises forty feet above the floodplain 

 of the Mississippi river. Worthen did not observe it on the Illi- 

 nois side of the river, five miles away, but a foot or two of it 

 is well shown there in times of very low water, when it appears 

 under the great massive sandstone which forms the bluffs at 

 this point. 



In lithological character the rock is a deep buff, somewhat 

 earthy, magnesian limestone, rather heavily bedded, and con- 

 taining some sandy material. It is apparently destitute of 

 organic remains. Being the lowest bed exposed along the 

 Mississippi between the Missouri river and northern Iowa, 

 it is of special interest; while within the area of exposure its 

 position is clearly indicated as immediately underneath the 

 great Cap-au-Gres sandstone. 



Gap-au-Gres Sandstone. — As typically developed at Cap-au- 

 Gres, the formation consists of a very massive, fine-grained, 

 soft sandrock, homogeneous in texture and white or yellowish 

 in color. Directly at the headland no strata rest upon it, but 

 a mile upstream, owing to the pronounced northward dip, the 

 overlying limestone begins to appear in the top of the cliff. 

 From the headland up to the ferry landing, a distance of per- 

 haps two miles, the sandstone forms high perpendicular and 

 castellated cliffs, which rise directly from the water's edge 

 (plate III). On the Missouri side of the Mississippi it forms the 

 middle and lower parts of the steep bluffs north of Winfield. 

 Fifteen miles northwest of the last named place, near the line 

 of the fault, it appears also to be exposed in the beds of two 

 small creeks. Worthen, in mentionirg this formation, corre- 

 lated it with the St. Peter sandstone of northern Illinois, but 

 that formation is probably not represented so far south. The 

 Missouri geologists referred it to the Saccharoidal sandstone. 

 It is regarded as the base of the Silurian. 



