358 Lever ett — Weathering and Erosion as Time Measures. 



conditions have been uniform and similar to the present con- 

 ditions. It is probable, however, that a period of aridity came 

 in at the time of the loess deposition, and this would lengthen 

 materially the time required for the development of the drain 

 age lines in the Illinoian drift. 



The presence of a drift sheet of Illinoian age in the Kee- 

 watin field is still open to question. The only deposits sus- 



Fig. 3. 



Fig. 3. Part of the Belleville, Illinois, quadrangle, showing Post-Illinoian 

 drainage under a fair stream gradient. Scale, 1 : 93700. Contour interval, 

 20 feet. 



pected to be of Illinoian age, so far as yet noted, are found in 

 southeastern Minnesota and neighboring parts of Wisconsin, 

 just outside the limits of the Wisconsin drift, and these have 

 not as yet been given sufficiently close study to clear up their 

 relations to the broad sheet of Illinoian drift of the Labrador 

 field. 



