620 FRANK LEVERETT 
of sediment. If to this is added the 35 feet of aggradation that is 
due to the lengthening of the stream, the full difference between 
the level of the rock bed and the level of the present river is 
accounted for. There thus seems no need for postulating disatro- 
phic movement. Farther up the river the rock barriers in the path 
of the stream, above Keokuk and above Rock Island, contribute 
in holding the present stream to a high level. 
GLACIATION IN ITS RELATION TO DRAINAGE MODIFICATION 
The preglacial Mississippi in its course from near Red Wing 
in southeastern Minnesota to the lower Illinois valley, and thence 
to St. Louis and Cairo, appears to have suffered little or no disturb- 
ance in the first two stages of glaciation, the Nebraskan, or pre- 
Kansan, and the Kansan. The Iowa branch of the preglacial 
Mississippi was more seriously disturbed by these early glaciations. 
Its valley was so completely filled as far down as Muscatine that 
its course is known only by borings. It was also completely filled 
for a few miles in the southeastern corner of Iowa. The filling as 
far down as the line of Iowa and Missouri is found to embrace the 
pre-Kansan as well as Kansan drift. In the vicinity of Fort 
Madison, Iowa, the pre-Kansan drift fills the old valley from the 
level of the rock floor, 135 feet below the present stream, to a height 
of about 75 feet above the river. A black soil marks its upper 
limit. It is overlain by Kansan drift, and this in turn by Illinoian 
drift, which rests on a soil developed on the Kansan drift. Other 
records in eastern Iowa, southern Minnesota and western Wis- 
consin show that valley excavation had reached its lowest limits 
prior to the earliest glaciation. One of the clearest records is at 
Washington, Iowa, brought to notice by Calvin.?. It is in a locality 
where no drift later than the Kansan is present, and is in the line 
of a western tributary of the preglacial valley utilized by the 
Mississippi below Muscatine. The Kansan drift extends to a 
depth of 115 feet, and rests on a peaty bed containing wood and 
cones of the black spruce. Below this is the pre-Kansan drift, 
extending to a depth of 350 feet from the surface, or to about 
4oo feet above sea-level. This level of the rock floor fits in well 
t American Geologist, Vol. I (1888), p. 28. 
