160 Sioux City Academy of Science and Letters. 
Todd* has suggested that much of the loess mate- 
rial may have been derived from western tributaries of 
the pre-Glacial Missouri, as well as from beds stirred up 
by the Dakota glaciers, with the possibility that these 
beds may have formerly extended eastward to the pres- 
ent James River Valley where the Missouri flowed in 
pre-Glacial times, and thus have been eroded and pushed 
southward by the action of the ice sheet. 
Whatever the origin and mode of deposition of the 
loess may have been it is evident that lying above the 
glacial till it covered the surface so thickly that much of 
the pre-Glacial topography was obliterated and the land- 
scape may have presented after the recession of the ice 
and the draining off of the swollen streams somewhat the 
aspect of a mud-covered plain with the great trough of 
the Missouri filled nearly to its brim with silt. Toward 
this nearly filled trough the plain sloped from the east 
and from the west, while the whole had a southerly slope. 
On this plain the post-Glacial drainage has worked, Mis- 
souri River rapidly re-eroding its trough down to the level 
of its present flood plain on account of having not much 
consolidated rock material to remove, while the minor 
streams have more slowly restored the old topography by 
working downward along and into the former lines of 
drainage. During recent time the formation of soil has 
continued through the action of weathering, erosion, veg- 
etation, the settling of dust, and the burrowing of ani- 
mals. Alluvial soil has been spread over the Missouri 
flood plain and short distances up those of its minor trib- 
utaries. The great river has frequently changed its me- 
andering course, even within the memory of inhabitants 
of this region. 
ECONOMIC GEOLOGY. 
Dakota County contains no metalliferous ores and 
but few nonmetalliferous minerals of economic value. 
CLAYS. 
Undoubtedly the Cretaceous clays of the county are 
the most important of these resources. Beyond ascer- 
aTodd, J. E., The Moraines of southeastern South Dakota: Bull. U. 
S. Geol. Survey No. 158, p. 99. 
