148 The American Geologist. September, 1902 
onlv a third, or, a- I think more probaljly, only al:)OUt an eighth, 
so old as the Hint hatchets ot St. Acheul and other localities of 
the old world. 
It will be objected, to my estimate of the antiquity of the 
Lansing- man, that the subserial erosion, weathering, and var- 
ious other features of the Kansan, Illinoian, lowan, and Wis- 
consin drift deposits, with the associated interglacial beds, ne- 
cessitate a much greater duration of these stages of the Glacial 
period than 1 have here suggested. Instead, I would reply that 
the old till sheets and the loess are spread somewhat evenly 
on the prcglacial rock surface, all the valleys and grand topo- 
graphic forms of the country in the southern part of our drift 
area being of preglacial origin. The drift earliest eroded from 
the preglacial lavicl was also largely the residuary clays and de- 
caAing rock of the surface, accounting for the great contrast 
in composition of the more southern and the more northern drift 
deposits, even when of the same mode of formation and the 
same age. 
Concerning the deposition of the loess along the 3iIissouri 
river and on the older drift to long distances at each side, I 
think that its derivation chiclly from the englacial and finally 
superglacial drift is clearly demon-strable. Swept by the sum- 
mer floods from the melting ice-surface, it was laid down as a 
deep valley drift deposit, thickening and lifting the great river 
on the vast floodplain until it flowed, during the hot part of each 
year, in a lakelike sheet of water, probably from 5 or 10 to 30 
or 40 feet deep and far wider than the present bottomlands, 
at the general hight of the loess bluft's and uplands. During the 
cool but not wintry parts of the year, in the spring and autumn, 
when the floods were reduced to comparatively small channels, 
or sometimes throughout several years together having less 
plentiful melting of the ice-sheet, land vegetation and air-breath- 
ing mollusks could occupy the newly deposited loess tracts. But 
the scantiness of vegetation on areas subject to summer over- 
flow permitted the winds to carry ofT much of this very fine loess 
and to spread it over the contiguous country in massive swells 
and ridges, conforming in a general way to the previous contour. 
With decrease in the suppl\' of both water and loess, when the 
lowan ice-melting was nearly finished, the rivers eroded deep 
and wide valleys in their loess plains, the valley of the Missouri 
