122 IOWA STUDIES IN NATURAL HISTORY 



from the Cretaceous. The latter seems quite probable for 

 the reason that the Cretaceous deposits in this district con- 

 tain pieces of gravel similar to the high-level gravel and 

 also chalky particles which appear in the stratified deposits. 

 The latter particles, however, could not have stood much 

 transportation and are of themselves a strong argument for 

 the local derivation of the high-level gravels. Other material 

 derived from the Cretaceous in the earlier stages of the dis- 

 section of the Cretaceous surface might have become wide- 

 spread as stream gra"\^els on the old surface of the pene- 

 plain. (2) It should be noted that although Winchell may 

 have been correct in his conjecture that certain gravels des- 

 cribed by him were locally derived from Cretaceous de- 

 posits, it does not follow as a corollary that the time of 

 derivation was Cretaceous and that therefore any surface 

 upon which these gravels lie is Cretaceous. In Winchell's 

 report^ he distinctly states his belief that the gravels which 

 he here describes were placed in position "by drift forces." 

 Whether he meant that they were Cretaceous deposits re- 

 worked during the Pleistocene period by a glacier is not 

 clear. Certainly they are not glacial, but undoubtedly they 

 were derived from the Cretaceous deposits revealed by post- 

 Cretaceous streams. Winchell seems to have believed that 

 at least some of the gravel deposits in Minnesota were of 

 post-Cretaceous age. The writer believes that the high- 

 level gravels of the Driftless Area, nowhere associated with 

 sands, silts, or clays, are post-Cretaceous, though perhaps 

 derived partly from Cretaceous formations containing 

 gravel layers or levels. (3) Winchell's conclusions were 

 with regard to local and isolated cases only and had no ref- 

 erence to gravel deposits of similar nature found in local- 

 ities not suggesting a local derivation. (4) The Dodgeville 

 plain constituting a stratigraphic base for the gravel has 

 never been traced and found to underlie Cretaceous rocks 

 but on the contrary its altitude in Minnesota is such as to 

 cause it to bevel the Cretaceous. (5) The gravels extend 

 far beyond any known Cretaceous aiid occur extensively 

 where there is not the slightest indication of Cretaceous age. 



l~Winchell. N. H., Gcol. u-iid Nafl Hist. Surv. Minn., Vol. I, p. 309. 



