136 S. CALVIN PRESENT PHASE OF PLEISTOCENE IN IOWA 



present that here are two sheets of drift recording two distinct stages of 

 glaciation, with abundant evidence of an interval between them. 



PKE-KANSAX OR 8UB-AFTONIA19 DRIFT 



The pre-Kansan is dark blue, almost black in color. It has a habit 

 peculiarly its own of breaking into small fragments or crumbling into 

 finer particles on continued exposure. At the classic locality near Alton 

 Junction this older drift is exposed in the west bank of Grand river, a 

 mile below the station, and is overlain by more than 30 feet of water-laid 

 gravels which were at one time worked for railway ballast. Overlying 

 the gravels is a heavy deposit of typical Kansan till. The same gravels 

 are exposed in a great ballast pit at Afton Junction, from which locality 

 came the name "Aftonian." given by Chamberlin to the gravels as well 

 as to the entire interval of which they form part of the record. Four 

 miles farther east near Thayer, is another pit of Aftonian gravels. At 

 all the localities named the Aftonian beds are covered with from 20 to 30 

 feet of characteristic Kansan (plate 1. figure 1). 



KAXSAX DRIFT 



The Kansan differs from the pre-Kansan physically. It is light blue 

 or gray in color when unweathered: it is cut by numerous intersecting, 

 vertical joints, and it breaks into large, irregularly shaped angular blocks. 

 That there are physical differences may be demonstrated wherever there 

 are opportunities for comparison, but a concrete case may be cited in the 

 section illustrated by Savage in his report on Tama county. 2 Here the 

 two older drifts are separated by a mere thin soil band. The lower, as 

 shown in the figure, is compact but relatively plastic, and the steam 

 shovel passing through it left the imprint of its cutting margin : the bed 

 above the Aftonian soil, hard almost as sun-baked bricks, and jointed 

 vertically, yielded to the force of the shovel by breaking into irregular, 

 prismatic blocks. Physical differences, constant and consistent, mark 

 these two beds of till as distinct (plate 1, figure 2 ). 



AFTOXIAX IXTERYAL 



The Aftonian was a real interglacial interval. It was a long interval, 

 but the data which might furnish a basis for estimating its actual or 

 comparative length are as yet wanting. With the possible exception of 

 the "forest bed" which McGee. in his early work on the Pleistocene, had 

 found everywhere between his '•'Upper and Lower till.'' the gravels at 

 Afton Junction and Thayer were the first of the Aftonian deposits to be 



Geology of Iowa. voL siii. page 231. 



