FEATURES OF FORT DODGE GYPSUM 



595 



There is little evidence to show the age of this solution siir- 

 face. In some places gray drift fills the hollows in the gypsum 

 while yellow oxidized till extends across hollows and eminences 

 alike, without curving down at any point. In one place an ox- 

 idized band bends up over the gypsum mound. There is no indi- 

 cation of slumping or settling of drift into the hollows as the 

 gypsum was dissolved away. If all of the drift here is Wiscon- 

 sin, as it seems to be, its condition and positioi would seem to in- 

 dicate that the solution was accomplished mostly in pre-Wis- 



Fig. 202. — Domes in the gypsum on an exposed surface up Two Mile creek. 

 Three feet of red shale overlies the gypsum. 



cousin (Peorian) time at lea.st, and it might, of course, be earlier 

 than that. The illustrations show that the pebl>le band and the 

 humus zone extend, for the most part, in uniformly straight 

 lines parallel with the surface of the ground. The fact that this 

 locality is on the upper slope of the valley wall makes escape of 

 the ground water easy and would nermit of relatively rapid pas- 

 sage of these waters through and over the rock. This condition 

 might point to a more recent date for the formation of this sur- 

 face. At the same time similar topographic conditions have pre- 

 vailed since the valley was formed in post-Kansan (Yarmouth) 



