EROSIONAL HISTORY OF DRIFTLESS AREA 111 



themselves. The renewed activity was felt last in the tribu- 

 tary valleys and the terraces have therefore been eroded 

 least there, so that they are now most conspicuous in val- 

 leys tributary to the main streams rather than in the main 

 valley itself. There are coarse stream-laid gravels on some 

 of these terraces, as for instance in the valley of Pine Creek 

 southwest of Richland Center, and in the valley of Honey 

 Creek near Plain. It is clear that these terraces also are 

 not significant in connection with the erosional history of 

 the general surface of the Driftless Area. 



Flat-topped terraces, consisting of alluvial and lacustrine 

 materials are found abundantly in nearly all the larger 

 tributary valleys to the Mississippi and Wisconsin rivers. 

 The origin of these terraces has been worked out- and found 

 to be due to a partial filling of the Mississippi and Wiscon- 

 sin river valleys by fluvio-giacial material at the time of 

 the Wisconsin ice invasion, the consequent ponding of the 

 tributaries, and the re-excavation of the fill in the mains 

 and the tributaries after the retreat of the Wisconsin 

 glacier. The origin of these terraces too has little to do 

 with the general erosional history of the Driftless Area as 

 a whole. 



Although there are many terraces in the Driftless Area, 

 it is concluded that none of them bears evidence of more 

 than one cycle of erosion in the region. 



Upland Fluvial Deposits (high level gravels) 

 One of the best evidences that there have been more than 

 one cycle of erosion in the Driftless Area is found in the 

 fact that stream deposits exist on some of the summit sur- 

 faces of the Area. These deposits have been known for a 

 long time and have usually been referred to as "high-level 

 gravels."' As the term implies, these deposits occupy the 

 highest portion of the topography and consist almost en- 

 tirely of gravel. 



1. Trowbridgre, A. C. and Shaw, E. W.. Bull. No. 26, ///. Geol. Surv.. pp. 145-15-> 



2. Strong. Moses, Geol. Wis.. Vol. IV. 1875-79, p. 88. 



Winehell. N. H.. Geol. and Xat'l Hist. Suri\ Minn.. Vol. I, 1884, pp. 305-310 ; 353-356. 

 Chamberlin. T. C. and Salisbury, R. D., Sixth Ann: Rept. U. S. Geol. Sun\, 1884- 

 85, p. 273. 



Salisbury, R. D., Bull. Geol. Sac. .Am., Vol. 3, 1892, pp. 183-186; .Jour. Geol., Vol. Ill, 

 1895, pp. 655-667. 



