28 TJie American Geologist. Januaiy, ]003. 
On the melting and drift-bearing- margin of the ice in the 
Missouri, Mississippi, and Ohio basins, where a great propor- 
tion of the drift is very fine-grained, with usually an appreciable 
calcareous element, from erosion of limestones or of the fre- 
quently calcareous shales of the great Cretaceous areas west 
of the Mississippi, the superglacial drift, washed by the streams 
of the ice melting, and of the accompanying copious rains, 
yielded to the rills, brook;:., and rivers discharged from the 
ice surface, a very abundant freight of fine muddy silt, usually 
somewhat calcareous. Being laid down in the ice-walled chan- 
nels of the streams near their debouchure to the open ground, 
or along- the edge of the icefields, this modified drift is pre- 
served to our time in the very instructive loess paha, and 
in the high loess embankments and slopes that fringe the outer 
boundary of the lowan till expanse. 
What conditions produced and controlled the deposition 
of the valley loess ? First, this region was differentially de- 
pressed, with all the North American area of glaciation, until 
it stood lower than now, the maximum reelevation in the Mis- 
sissippi basin being, as I think, about 500 feet, or, according 
to the estimate of Chamberlin and Salisbury, probably as much 
as 800 feet. (The physical characters of the earth crust, and of 
the plastic or molten interior, which could admit such dif- 
ferential movements. I discussed eight years ago in Mono- 
graph XX\' of the United States Geological Survey, entitled 
"The Glacial Lake Agassiz.") Second, with restoration of a 
temperate climate on the border of the ice-sheet, brought by 
the subsidence from the formerly high elevation, extensive 
surface melting of the ice exposed its englacial drift, much 
of which was washed aw a}' and borne into the vallevs beyond 
the ice boundary. 
The very fine modified drift in the ^Mississippi region, 
called loess, could be carried by the gently flowing streams 
far along the great valleys, in which it formed thick fioodplain 
deposits, the swollen rivers being uplifted on them to bights 
of 150 to 250 feet above their present beds. Today the ]Missouri 
is sometimes called "the Rig Muddy," a name which it well 
deserves, for at all seasons a white plate lowered into its 
muddv current becomes invisible within a depth of two or three 
feet. But 'during the final melting of the ice-sheet this river 
