59° 



SAMUEL CALVIN 



to show the constant relation of the prevailing gravels of the region 

 to the typical Kansan drift. The same relation is shown in the 

 fine Pleistocene section which occurs a few rods north of the Mitchell 

 County line, not far from the southwest corner of Section 14, 

 Township 97, Range 17. Here, in the south bank of Rock Creek, 

 is an exposure of typical Kansan, blue in color and breaking into 

 the characteristic polyhedral blocks, with an exposed thickness of 

 50 feet; at the top is a discolored, weathered zone three to four 

 feet thick; next in order is a gravel bed, rusted and rotted, thick- 

 ness about two feet; and all is covered by a thin loamy deposit 

 carrying many fresh bowlders of varying size, belonging to a post- 

 Buchanan stage of glaciation — the Iowan. 



But it is certainly unnecessary to offer additional evidence along 

 this line. Cases of the kind already cited may be multiplied 

 indefinitely. Fortunately the Buchanan gravels are especially 

 well developed in northeastern Iowa, and in the Iowan area they 

 uniformly afford indubitable evidence of a younger, newer, later 

 stage of ice invasion. Outside the Iowan area, as at Colesburg, 

 Delaware County, on the east, and at Iowa City on the south, the 

 Buchanan gravels are covered with heavy deposits of loess, and 

 without the least suggestion of later glaciation. Some very impor- 

 tant event, later than the deposition of the gravels, an event which 

 caused the deposition of a body of till ranging up to 20 or 30 feet 

 in thickness and carrying bowlders more than 12 feet in diameter, 

 occurred within the Iowan area and did not occur outside of it. 

 What was that event ? Observations in and around the area lead 

 unavoidably to but one conclusion, a conclusion that admits of 

 no question : 



The Iowan drift is. 



The Iowan Drift Is Young as Compared with the Kansan 

 The superposition of the Iowan till and the great Iowan bowlders 

 on the weathered Buchanan is all the evidence needed to demon- 

 strate that the Iowan is younger than the Kansan. The freshness 

 of the granites in and on the Iowan — many with the sharp angles 

 caused by fracture unaffected by weathering (Fig. 10), while the 

 granites of the Buchanan are very largely in an advanced stage of 



