574 PROCEEDINGS OF THE SAINT LOUIS MEETING 



wells near Nora post-office, in the southeast corner of township 94 north, range 

 50 west, where the top of the Benton is found to be about 1,380 feet above tide. 



Another area of Benton immediately below the drift has been quite confidently 

 determined in the Minnesota valley. Minnesota geologists had inferred from 

 numerous Pierre fossils found in Lyon county, Minnesota, that the formation 

 underlying the drift there is Pierre. This inference was extended to the region 

 around Big Stone lake, partly because no flow was obtained from a deep boring 

 at Milbank, South Dakota. 



More careful inquiry the past season developed the following facts: 



1. Chalk of bluish white tint and containing angular fragments of Inoceramus 

 shells like the Niobrara was struck in wells in a strip extending from section 2, 

 township 118 north, range 48 west, 10 to 12 miles in a northwest direction. It 

 was reached at from 50 to 80 feet depth, and in some cases was found to be 20 to 

 25 feet thick. Its altitude is about 1,100 feet above tide. 



2. At Milbank, in several wells, shells of Prionocyclus were found at a depth of 

 about 40 feet from the surface in black clay. One examined was about 10 inches 

 across and in a concretion which showed no abrasion. It is probable that the 

 clay was preglacially disintegrated from Benton shale. 



3. In the deep well at Milbank a strata of copious soft water was struck at about 

 300 feet, which rose to 20 feet of the surface.. Apparently the same was struck at 

 200 feet, 1 mile south, and a few miles southwest flows of similar water are obtained 

 from about 400 feet, all probably from the Dakota. 



4. No exposures of shale were found on Big Stone lake, except on the Minne- 

 sota side near the north end, where there is a slope of bare lead-colored shale with 

 biscuit-shaped ferruginous concretions with cracked interior. It shows 25 feet 

 above the water and appears to be part of a slide from 20 feet higher. No fossils 

 were found. Similar clays have been observed farther north.* 



5. At White Kock, in the extreme northeast corner of South Dakota, according 

 to Morley and Steinmest, well drillers, blue chalk 20 feet thick was struck at a 

 depth of 300 feet. If this is Niobrara it would indicate quite a drop from near 

 Revillo. That fact, with its thickness corresponding to that of the Greenhorn, 

 makes it seem likely it is the latter. 



Hence we may reasonably infer that the Benton underlies the drift in that 

 region east of a line running north and south a few miles west of Milbank and 

 curving south and east past Revillo. The limit extends north near Browns valley. 

 The eastern limit is probably bounded on the east by granite knobs like those 

 exposed from Bigstone lake to Granite falls. 



Occurrence in adjacent States 



In Minnesota, Benton fossils have been found in the shale at Sauk rapids. In 

 Lyon county the Pierre fossils reported by the Minnesota survey were certainly 

 from the Pleistocene till in some cases, and probably in all, and not far away 

 copious soft water obtained from 250 feet rises within 20 feet of the surface, and 

 strong flows come from 350 to 400 feet. If the Benton has its usual thickness 

 there, it doubtless lies just below the till. 



In Iowa, along the Big Sioux, the Greenhorn forms a marked terrace of erosion 

 as far north as Akron. The Carlile clay is deeply developed above it at Ha warden, 

 and is 60 to 75 feet thick as far south as east of Elk point. 



"•Minn. Geol. Survey, vol. i, p. 619, 



