EROSION AL HISTORY OF DRIFTLESS AREA 59 



mile. Four computations in the area north of the arch 

 show an average dip there of N. 35° W. in direction and 6.9 

 feet per mile in amount. 



Fie 11 Sketch map of the Driftless Area and its environs showing structure con- 

 tours on the contact between Prairie du Chien and Jordan formations. bU-uctural 

 contour interval equals 100 feet. Horizontal scale 1/500,000. Contours are broken 

 where courses are conjectural. 



