22 
The American Geologist. J"'-^' ^^oi. 
paratively old, for the valleys occupy a large part of the sur- 
face, are of the basin tvoe, and most of the ridges have been 
reduced much below the original level. 
Where the Illinoian ice sheet left the surface as a gently- 
sloping till plain as in western Illinois and southeastern Iowa, 
we find a distinctly different type of topography. Valleys are 
fairly numerous and in places lOO or 200 feet deep, but they 
are of the canon type rather than of the basin t-v-ne so charac- 
teristic of the Kansan areas. j\Iuch of the surface has been re- 
duced but little below its original plain level and the railroads 
in crossing Illinoian areas transverse to the main drainage lines 
are most of the time on the plain so that the passenger may 
leisurely study the upland country while on the Kansan areas, 
as already mentioned, he catches but a fleeting glimpse of the 
old plain level as the train crosses a main divide. Surely this 
difference means something. Probably of the total amount 
of the Illinoian drift between the level of the main valley bot- 
toms and the original plain there has been removed by erosion 
no more than one-tenth or at most one-fifth. 
Where the Illinoian and Kansan areas adjoin as in Lee 
county, Iowa, the contrast in topography is great. The very 
thoroughly dissected and much eroded Kansan drift is over- 
looked b}' a broad ridge of Illinoian drift, whose comparatively 
steep slopes, if exposed to erosion as long as has been the former 
district, should be deeply trenched by valleys ; indeed, the 
moraine should be nearly destroyed. For about one mile, just 
outside or west of the moraine, the Kansan erosion topography 
is mostly obscured, being buried under a sheet of silty material 
of Illinoian age. It is here evident that the Kansan area had 
largely received its present old erosion topography before the 
Illinoian drift was deposited on it. 
In southeastern Iowa the streams in the Kansan area did 
not cut down to so great depth as in Missouri, the larger pre- 
Illinoian valleys apparently having had a depth of only 50 or 
60 feet, though they had a width of one or two miles. On 
page 106 of his admirable monograph on the "Illinois Glacial 
Lobe," Mr. Leverett says : "In the district occupied by the 
Kansan the erosion is so great that only narrow remnants of the 
original drift plain are preserved, along the water partings. But 
in the district occupied by the Illinoian more than half of the 
