Valley Loess of Lansing, Kan. — Uphani. 33 
along the valleys and deposited much modified drift. They 
descended with the same valley slopes as now, and their strong 
currents bore the finest silt far southward, to the lower Mis- 
sissippi and the Gulf, while the gravel and sand were deposited 
along the upper part of the valleys. In the close vicinity of 
the moraines the outwashed coarse modified drift fidled the 
valleys 100 to 200 feet, or more, above the present rivers ; but 
the fine clayey silt borne farther down the valleys was deposited 
in much less volume than the earlier loess, being mostly swept 
on bv the river floods to the Mississippi delta. Along the Big 
•Sioux valley, on the northwest boundary of Iowa, a floodplain 
of modified drift associated with the moraines has an average 
width, as described by F. A. Wilder, of one and a half miles, 
and is about ten feet above the present relatively insignificant 
bottomland, which averages only about a fifth of a mile in 
width.* Below the junction of the Big Sioux with the Mis- 
souri, this tloodplain of Wisconsin time continues with a width 
of 6 to 12 miles on the east side of the Alissouri through the 
distance of 90 miles to Council Blufl^s and Omaha, having 
only the same slight altitude above the river. Southward from 
the mouth of the Platte river, as I think, the old Wisconsin 
floodplain was lower than the bottomland today, which has 
evidently gained in depth, rather than lost, ever since the Ice 
age. Conditions requisite for silt deposition 30 to 50 feet 
above the Missouri at Lansing appear thus not to have existed 
since the lowan loess was spread deeply along this valley. 
It seems needful to add only a few words more. The an- 
tiquity of the Lansing man I think to be measured by about 
12,000 years, or, at the longest 15,000 years; and this is prob- 
ably the earliest evidence of man known in relation with depos- 
its referable to glaciation in America. The stone arrowhead not- 
ed by Willistonf as found with the bones of an extinct s])ecies 
of bison in western Kansas is undoubtedly about equally old. 
But the stone implements and human bones found by Ab- 
bott, Putnam, Volk, and others, in glacial gravels at Trenton, 
N. J., belong to a later stage in the departure of the continental 
ice-sheet. Probably almost contemporaneous with these are the 
evidences of early man made known in modified drift deposits 
* Iowa Geol. Survey, vol. x. 1900, p. 142. 
f The Unversjtv Geological Survey of Kansas, -vo]. ii, 1S97, po. 297-308. 
Am. Gkologist, vol. xxx, pp. 313-3in, with section, Nov., 1902. 
