172 IOWA ACADEMY OF SCIENCE 



gray through buff to brownish within ten feet or less shows 

 how brief, relatively, has been the period during which tins 

 sheet of till has been exposed to the elements- 

 It will be noted that there is in the first section no trace of 

 a drift beneath the loess. All of this had been swept away and 

 the Coal Measures shales leveled off and a layer of geest formed 

 before the loess was deposited. The cuts indicate also that the 

 Wisconsin drift in turn was spread out on a mature topog- 

 raphy developed on the older surface. 



Iowa Geological Survey, 

 Des Moines. 



