14 s S. CALVIN" PRESENT PHASE OF PLEISTOCENE IN IOWA 



of mature erosional topography has been developed. There are no un- 

 drained upland areas: the rain-cut trenches have invaded every part of 

 the original plateau. In parts of southwestern Iowa the rivers — for 

 example, the Xodaway, at Hepburn, in Page county — have eroded vallevs 

 in the Kansan drift 200 feet in depth and 3 or 4 miles in width. At 

 Iowa City the Iowa river has made a valley SO feet in depth since the 

 Kansan. and half of this depth has been cut in Devonian limestone. All 

 the streams of the Kansan area and practically all parts of the Kansan 

 surface tell the same consistent story; all record a long period of active 

 erosion. Along the Iowan-Kansan border there are many points where 

 the differences are strikingly brought out. On one side of a fairly defi- 

 nite line the topography is water-carved : on the other side it is ice- 

 moulded ; on one side a series of rounded, billowy, loess-covered ridges 

 fitting into a .dendritic system of broadly open, erosion-cut ravines : on 

 the other a great boulder-dotted plain, untouched by erosion, over exten- 

 sive areas as level as a floor. The amount of erosion over the general 

 surface of the Kansan is many times as great as that over the general 

 surface of the Iowan. If differences in the amoimt of erosion may be 

 taken as a fair measure of the differences in the age of two drift sheets, 

 then the Kansan is certainly a hundred times as old as the Iowan. If the 

 two drifts be compared with reference to the magnitude of the changes 

 brought about by weathering, a similar conclusion is reached. The great 

 thickness of the altered and oxidized zone in the Kansan was noted in 

 making comparisons with the Illinoian. and reference has also been made 

 to the inappreciable amount of weathering in the Iowan. To say that 

 weathering in one case is one hundred times as great as in the other is 

 to make a very conservative estimate. If weathering is a measure of age. 

 we may repeat the statement already made, that the Kansan is certainly 

 one hundred times as old as the Iowan. 



Fourth interglaciar Interval, the Peorian 



The Peorian interglacial interval, which followed the Iowan ice stage., 

 was very short, and at its close a fifth ice-sheet, coming from the Kewatin 

 center, flowed into Iowa and distributed the body of till called the Wis- 

 consin. As in the case of the Iowan. it was only a terminal lobe of the 

 Wisconsin that crossed the Iowa-Minnesota line. In the main, the Wis- 

 consin lobe lies to the west of the Iowan ; it overlaps the Iowan for some 

 distance along its eastern edge : in general it overlies loess-covered Kan- 

 san. The southern extremity of the Wisconsin drift lobe is at Des 

 Moines, and within the city limits excavations for various purposes have 

 o-iven sections showing (1) profoundly altered, oxidized and weather- 



