ORIGIN 307 



which are pot-holes a foot or more in diameter. The presence of these 

 proves a rapids or falls which it is rather difficult to believe were made 

 other than at the time the gravels were deposited. At Waukon, Iowa, 

 test pits are stated to have found the ore to be separated from the under- 

 lying dolomite by a foot or two of residual clay. Unworn fossils and 

 boulders from the underlying rock are present in the iron oxide. The 

 stream at that place was not so swift as at Devils Lake, and bog ore could 

 deposit against banks of dolomite. At Seneca the conglomerate pene- 

 trates 65 feet into a fissure in the bedrock. Whether it came into this 

 fissure during or since its origin can only be conjectured ; but, considering 

 the generally im^jervious character of this material where iron oxide is 

 present, it is more probable that the deposit was washed into a fissure at 

 the time of deposition. At Mitchell, Iowa, the Windrow formation fills 

 an erosion channel in fresh Devonian limestone. In Guthrie County, 

 Iowa, the surface of the underlying Coal Measures beneath the Cretaceous 

 is rarely seen, but appears to be one of comparatively small relief. 



CONCLUSIONS AS TO ORIGIN 



The surface on which the patches of the AVindrow formation rest bevels 

 across the Huronian and Paleozoic strata, descending from 1,440 feet at 

 Devils Lake to about 1,300 feet along the Mississippi, about 1,100 feet in 

 northern Iowa, and, if the conglomerate of southwestern Iowa is a part 

 of the formation, it descends in that locality to about 1,050 feet. The 

 quartz pebbles were probably derived from the north or northeast of their 

 present distribution. It is suggested on subsequent pages that the gravels 

 were deposited during the Cretaceous period. The Cretaceous sea lay to 

 the south and west. The pebbles are believed to be of stream deposition 

 in a region of considerable relief, and to have been deposited by streams 

 flowing to the south and southwest. The absence, of cherts containing 

 Mississippian and Pennsylvanian fossils is in harmony with this conclu- 

 sion, since such most certainly would have been among the pebbles, had 

 the streams flowed in the opposite direction. 



Age of the Windrow Foemation 

 previous opinions 



With slight reservations, Winchell and Upham ascribed the gravels and 

 associated deposits of southeastern Minnesota to the Cretaceous (30, 31, 

 32, 33). Salisbury considered that "it is not beyond the possibility that 

 some of the beds . . . are Cretaceous, while others are Tertiary" ; but 

 that "the balance of evidence seems to favor'' reference to the latter (20). 

 Howell concluded that "it [the plain on which the gravels occur] may be 



