744 THE JOURNAL OF GEOLOGY. 



which is not altogether lost, is greatly obscured. A distinct innovation 

 is suggested in the chronological classification of the drift. Instead of 

 referring to a given drift deposit as belonging to the first, second, or 

 third glacial epoch or episode, it is proposed to designate the deposits 

 made during the more distinctly marked stages of the glacial period, by 

 the names of type localities. Thus it is proposed to apply the name Kan- 

 san to the deposits made by the most extensive sheet of ice. This 

 formation is now uncovered only along the southern border of the drift. 

 As now exposed, it finds extensive development in Kansas, Missouri, 

 Illinois, Iowa, Nebraska, and Dakota, and lesser development in several 

 other states. In general, this formation is thin at its outer edge, its 

 terminus not being marked by morainic accumulations. It has suf- 

 fered much erosion. In many regions, remnants only have escaped 

 destruction at the hands of erosive agencies. The rock surface under- 

 lying the exposed part of this formation was in general little modified 

 by the ice. 



The Kansan formation is overlapped by another sheet of drift, the 

 East lowan, which encroached upon it from the north, leaving only its 

 southern margin exposed. Between the two formations there is a wide- 

 spread body of soil, which is in many places thick. It was probably as 

 well developed as the soils of the present surface. It is known to extend 

 fully fifty miles back from the outer border of the East lowan forma- 

 tion. The plant remains which this soil contains have not been studied 

 in great detail, but they are such as to indicate a temperate climate. 

 The interval of deglaciation therefore was important. This interval of 

 deglaciation is called the post-Kansan interval. Some estimate of its 

 length is also based on the amount of erosion which the Kansan forma 

 tion has suffered, compared with that which has affected the next suc- 

 ceeding formation. 



Like the Kansan, the East lowan formation was once widespread, 

 but as a rule only that part of it which was not covered by later drift 

 can now be certainly differentiated. Like its predecessor the East 

 lowan formation is not generally bordered by distinct terminal 

 moraines. With the East lowan formation, the main body of the loess 

 seems to have been connected in time of origin. 



Following the East lowan formation, it would appear that there was 

 an interval of deglaciation sufficiently long " to permit a notable change 

 in the configuration and conditions of the land — the development of 

 capacious valleys ; the general carving of the surface into an erosion 



