572 CHARLES KEYES 



least on the surface of the ground. On the other, or north, side of 

 the anticline the same belts recur, as already stated. 



Bearing in mind the geographic position of this marked anti- 

 cline, an arch between the center of which and the bases of its limbs 

 there is a stratigraphic interval of more than 5,000 feet, it is quite 

 obvious that the Paleozoic belts originally did not really terminate 

 against it in southern Minnesota but rather extended over it and 

 were continuous with the similar Canadian belts. This being the 

 case, it is equally obvious that the lowan belts should not only not 

 stop against the arch, but should continue westward along the 

 strike of the fold, but beneath the Cretaceous covering. This is 

 found actually to accord with recently observed facts. A cross- 

 section (Fig. 2), which is drawn to scale, indicates the actual amount 

 of tilting displayed at the present time, with the part originally 

 laid down, but removed during Mid- Cretaceous times, represented 

 by dotted lines. 



There are incontrovertible proofs fixing within very narrow 

 limits the geologic date of the uprising of the great Siouan arch, 

 and also the time of its complete reduction again to an even plain 

 lying but little above the level of the sea. Since all of the Paleozoic 

 formations, from Cambrian to latest Carboniferous, take part in 

 the folding while the Cretaceous strata do not, it is manifest that 

 the main movement occurred in Early Mesozoic times, largely dur- 

 ing the Triassic and Jurassic periods. Comanchean time (Early 

 Cretaceous) in the region must have been principally a period of 

 rapid, enormous, and very complete denudation, since by the begin- 

 ning of Mid Cretaceous time, when marine deposition over this 

 part of the continent took place, the floor upon which the sediments 

 were laid down was as even as any known peneplain. The Creta- 

 ceous floor is a true plain worn out on the beveled edges of the 

 Paleozoics and older rocks. 



The areal distribution and attitude of the Cretaceous beds 

 along their eastern margin is suggestive. In Minnesota the western 

 half of the state is occupied by deposits of Cretaceous age. Outliers 

 occur far to the eastward — to the Mississippi River and the Mesabi 

 Range. The great thickness of Cretaceous deposits in the east- 

 ward-facing escarpment of the Duck and Riding mountains, in 



