214 
IOWA ACADEMY OF SCIENCE 
sinuous gorge of the Oneota, or Upper Iowa river in the western 
part of Allamakee county. 
The episode of energetic erosion was preglacial. The work ac- 
complished by the great river during its progress and before the 
episode came to an end is represented by a rock-walled gorge 
seven, or eight, or nine hundred feet in depth and four to six miles 
in width. This old preglacial gorge was deeper than the present 
one, for soundings, as at Clinton and many other points along the 
river, show that the channel has been filled with sand and clay 
and other detrital material to a depth of from one, to two hundred 
feet. This filling is probably the work of glacial time. Before the 
glacial period, however, the valley had been widened by recession 
of its walls, the bluffs rose from the level of the water in gentle, 
rounded slopes, the lateral valleys were also broadened, the topo- 
graphic features of the region were practically mature. 
Some of the changes wrought in the features of the Mississippi 
river by the earlier ice sheets, especially by the Kansan and the 
Illinoian, are recorded in the portion of the valley between Clinton 
and Keokuk; but there are no clear records of any important 
changes, referable to the earlier glacial stages, in that part of the 
gorge lying between Lansing and Dubuque. On the other hand, 
the latest stage of glaciation, the Wisconsin, has left its impress 
on the valley from Dubuque northward, in a multiplicity of records 
of unusual interest. The Wisconsin ice invaded the upper part of 
the drainage basin of the great river. While the glaciers of this 
stage did not extend as far south by several hundred miles as some 
of their predecessors, the volume of ice seems to have been very 
great, so much so that, during the period when the ice was melting, 
enormous floods of water poured along all the drainage courses 
which connected in any manner with the ice margin. Some or all 
of the streams were loaded to the limit of their capacity with sand 
and gravel. The Chippewa and Wisconsin rivers deserve especial 
mention on account of the vast quantities of material which they 
carried and discharged into the central drainage channel. The old 
gorge of the Mississippi, widened as it was by weathering and 
recession of the bluffs until it was far beyond the necessities of the 
preglacial stream, was now overtaxed by the great Wisconsin 
floods. 
In the normal development of every river valley the spurs and 
headlands between the lateral gulches assume more or less gentle 
slopes and become rounded and sodded over as the valley reaches 
maturity. The line marking the foot of the bluffs on both sides 
