PLEISTOCENE HISTORY OF MISSISSIPPI VALLEY 619 
extension in the past 40o years. The time since glaciation came 
in to displace the streams from their preglacial beds is likely to be 
at least 1,000 times, and perhaps 2,000 times, as long as the 4oo 
years involved in the growth noted by Upham. The extension 
may, therefore, be as great as from the site of Vicksburg, and 
perhaps from farther up. With this lengthening there would come 
a corresponding aggradation of the valley causing the stream to flow 
at a level materially higher than its rock bed. Studies by the 
present writer near Osceola in the northeast part of Arkansas have 
shown that the flood plain has been built up about 35 feet by a 
sediment finer than the underlying sand presumably in Pleistocene 
time. So the aggradation there appears to have been at least that 
amount, and it would be fully as great farther up the valley. 
Another condition that affects the Mississippi Valley below 
the mouth of the Missouri River, results from the greater amount 
of sediment now brought in by that stream and carried down the 
Mississippi, as compared with that likely to have been carried by 
it before its watershed became so extensively covered with loess, 
for loess furnishes the main part of the sediment. With increase 
of load a stream raises its gradient. The Missouri has a fall of 
about 300 feet in its lower 300 miles, while the section of the 
Mississippi which is less heavily loaded with sediment has a fall 
of only about 150 feet. Below their junction the stream has a 
slightly higher rate of fall than that of the Mississippi above the 
junction, it being 7.2 inches per mile from the mouth of the Missouri 
to the mouth of the Ohio. The effect of this load of sediment is 
clearly brought out by comparing this section of the Mississippi 
with the section of the Ohio immediately above its mouth. The — 
rate of fall in the lower 200 miles of the Ohio is only about 3 inches 
per mile, or less than half that of the Mississippi; yet the Ohio is 
a smaller stream, and should have a higher gradient were it carry- 
ing a similar load. ‘There is a difference of about 70 feet between 
the fall of the Mississippi and of the Ohio in the 200 miles above 
their junction, and this may give a measure of the amount of 
aggradation at the mouth of the Missouri, due to the heavy load 
- t“Growth of the Mississippi delta,” American Geologist, Vol. XXX (1902), pp. 
103-11. 
