88 PHYSICAL GEOGRAPHY. 



common granite boulders, from the northern to the southern 

 boundary of the State. 



In that portion of the middle third of the State, extending 

 from its northern boundary to the city of Des Moines, 

 occasional boulders and transported masses of light yellowish 

 magnesian limestone are found. These are usually rare, but 

 they have been found sufficiently numerous in some places to 

 serve the first settlers with material for limited quantities of 

 lime. 



With the exception of the quartzite and limestone boulders, 

 all others of the drift of Iowa are of some of the varieties of 

 granite, usually a reddish syenite. Even this variety varies 

 considerably by the varying proportions of its characteristic 

 minerals with occasionally slight additions of mica. 



The Drift Deposit varies much in thickness in different parts 

 of the State. This difference is partly due to the original 

 deposition, and partly to subsequent erosion. It is thickest 

 all along the dividing ridge before described, which consti- 

 tutes in great part the watershed between the two great rivers. 

 Along this ridge it evidently reaches a depth of not less than 

 from one hundred and fifty to two hundred feet. It is known to 

 reach a hundred feet in thickness in many other parts of the 

 State, and with few exceptions, it covers the surface everywhere 

 so deeply that wells or other artificial excavations very rarely 

 reach the stratified rocks, unless made upon a valley-side. 

 For this reason we have no present means of knowing what 

 the real depth of the deposit is over a great part of the State. 



There are two principal regions in the State where the Drift 

 Deposit is comparatively thin. One of these regions is trav- 

 ersed by the Shellrock river, from the southern part of Worth 

 county to the confluence of that stream with the Cedar ; and 

 the other is that portion of southwestern and western Iowa 

 which borders the Missouri river, and within which the Bluff 

 Deposit rests upon the accumulation of drift. It is true that 

 the underlying strata are there covered deeply from view, but 

 it is mainly the Bluff Deposit, and not the drift, that covers 

 them. 



