EROSIONAL HISTORY OF DRIFTLESS AREA 121 



The Age of the Dodgeville Plam 

 There is no known way to determine the age of the 

 Dodgeville plain by a study confined to the Driftless Area. 

 The most promising method lies in the attempt to determine 

 the age of the high level gravels which are contemporane- 

 ous with the plain. In 1882 Winchell^ discovered silts and 

 sands and clays of undoubted Chetaceous age in southeast- 

 ern Minnesota, and in the same district he found a gravel 

 deposit later found to be similar in some respects to the 

 hige-level gravel of the Driftless Area. Because the gravels 

 were associated with the Cretaceous deposits Winchell 

 tentatively assigned a Cretaceous age to them. In 1895, 

 after study of the Tertiary gravels of the Gulf Coast and 

 of Arkansas and southern Illinois, and after seeing the 

 gravel deposits of the Driftless Area at Seneca and Devil's 

 Lake, Salisbury- concluded that the gravels were not older 

 than Cretaceous nor younger than Lafayette, and was in- 

 clined to believe that they are late Tertiary in age. For 

 unstated reasons most recent writers have followed Win- 

 chell and tentatively assigned a Cretaceous age to the high- 

 level gravels and the plain on which they lie. Perhaps the 

 reason is that marine Cretaceous rocks in western Iowa 

 and Minnesota lie on a base of slight relief and bevel the 

 same southwest dipping formations as occur in the Drift- 

 less Area. 



After having spent part of a field season in southeastern 

 Minnesota, assisted by Professor Leroy Patton, the writer 

 is strongly inclined to favor the Tertiary age of the plain, 

 for the following reasons : (1) The gravels of the Driftless 

 Area are dissimilar froni the rocks which carry Cretaceous 

 fossils in Minnesota and from certain deposits of stratified 

 gravels in Minnesota believed to be related to or derived 

 from the Cretaceous rocks. Near New Ulm, Brown County, 

 there is an exposure of stratified high-level gravels inter- 

 bedded with sand and clay. The surface rock at this place 

 is regarded as Cretaceous. The stratified deposit itself may 

 be Cretaceous but might easily have been locally derived 



Winchell, N. H., Geol. and Nat'l Hist. Surv. Minn., Vol. I, pp. 309-310 ; 353-356. 

 Salisbury. R. D., Jour. Geo!., Vol. Ill, pp. 655-667. 



