IOWA ACADEMY OF SCIENCES. 15 



The well iit Eniiuetsburtr ! find reported by Prof. N. H. 'Winchell from notes Ijy 

 the borer Mr. Swan, in the Minnesota report for 1S79. From it we derive the 

 following: 



Two hundred and twenty feet — Drift and cretaceous clays. 



One hundred and nine feet — Sand, dark above, g^ray below. (Dakota.) 



Twenty-two feet — Red marl. .lurasso-Triassic. 



Thirty-two feet— Broken and sandy limestone. 



Four feet— Black shale. 



Thirty feet— Limerock. 



Fifteen feet— Gray shale. 



Two hundred and twenty-four feet — "Mapfnesian limestone." 



Ninety-five feet — Shales gray and blue. 



One hundred and seven feet — White sandstone, "St. Croix." 



Six feet — "Granite" (quartzite?). 



Besides these wells which reveal the depths, much additional light has been 

 derived from numerous exposures along the Big Sioux and Missouri rivers. There 

 are very few elsewhere. The Drift has buried the older rocks almost everywhere 

 else. 



The following generalizations are offered tentatively: 



1. There seems to be a slight development of the later Tertiary just below the 

 drift. This is found in the shape of fine sands found in the Le Mars well and 

 more clearly in the high sand pits which are opened 4-6 miles northwest of Sioux 

 City along the bluffs of the Big Sioux, 160-180 feet above the stream. No fossils 

 have been found in them, but the absence of northern erratics and their horizontal 

 stratification indicate their age to be older than the Ice age. Laminate clays and 

 similar sands are found east of Canton, S. D., on the Iowa side of the Big Sioux. 



2. The chalky beds of the Cretaceous are usually uppermost through the 

 region. I have not been able to trace a definite horizon very widely but the 

 following sumtoits of exposures determined by barometer from adjacent railway 

 station, may be helpful: Dakota City, Neb., 1251; St. Onge's, Iowa, 1255; Ponca, 

 Neb., 1245;" Hartington, Neb.. 1-324; Yankton, S. D., 1240; Scotland. S. D., 1275; 

 Volin, S. D., i:!00; Medicine Knoll, S. D., 1330; Akron, Iowa, 1175; Canton, S. D.. 

 1290; Brandon, S. D., 1315. 



The most complete natural section of the rocks, as was pointed out by Dr. 

 White in his report. Vol. 2, p. 196, is at Cedar Bluffs. The thickness of the chalk- 

 stone, or "Inoceramus beds," is 45-50 feet. Instead of repeating, I will refer the 

 reader to the sections well given by Prof. St. John in the report just mentioned. 

 The Cretaceous dips to the north so as to drop below the Big Sioux a little above 

 Akron, but it reappears near Canton and is still higher a little above Brandon, 

 S. D.. where it may be seen in place only a few feet above the red quartzite. 



The Benton clays, or the upper part of the Woodbury shales of the Iowa geolo- 

 gists, are 80-90 feet thick. 



3. The Iowa geologists divide the strata differently from Dr. Hayden. The 

 latter seems to make the top of the sandstone at the foot of the bluff at Sioux 

 City the division between the Benton and Dakota, and this horizon passes below 

 the river on the west side at Ponca. There is much shale below this, 24 feet 

 exposed at Cedar Bluffs and estimating from the relations at Sioux City as shown 

 by the well, we may calculate to extend over 120 feet lower, before it comes to the 

 continuous sandstone which the Iowa geologists have called Ni.shnabotna sand- 

 stone. The divisions of the former seem to correspond better with the lithologica 

 characters of the beds. 



