G. K. Warren— Valley of the Minnesota and Mississippi. 427 
by tributary sediment after the great volume of water from the 
innipeg basin had disappeared. 
Anomalies of Rock Island Rapids and Des Moines Rapids.—At 
Rock Island the river has left the ancient valley, which just 
below Rock River seems to be lost. e might have supposed 
that it ended here but for finding it again below Muscatine and 
continuous down to the Des Moines Rapids, where it is lost 
again. Just below, however, we find it again, and it then is 
continuous until it widens out into the broad expanse below 
the junction of the Ohio, although the river again leaves the 
main valley, without sufficient apparent cause, at Fountain 
Bluff and at the Grand Chain. 
e had not time to study out where the course of the ancient 
valley was between Rock Island and Muscatine, but at the Des 
Moines Rapids we were more fortunate. 
The following description of this vicinity and Diagram E will 
present the points that appear deserving of consideration. 
The river, as it passes the town of Madison on its right bank, 
which is there 150 feet high, washes against a bluff composed 
of clay and sand arranged in a manner resembling “the ebb 
and flow structure.” Mr. Worthen gives a good description of 
this formation in the Iowa Geological Report, volume i, part 1, 
page 187. This kind of bluff is seen only on the right bank, 
and extends continuously a distance of about twenty-five miles 
from the mouth of Skunk River down to the point D (see Dia- 
gram K), where it joins the Keokuk limestone rock just back of 
Montrose. 
In descending the river from Madison, the river gradually 
recedes from this bluff, and a few miles above D the bluff is 
three miles from the river. A large portion of the intervening 
Space is occupied by a sand terrace, varying from twenty to 
fifty feet in height above the high water; the other portion is 
bottom land, subject to overflow. ieee 
The river, before reaching Montrose, has a width within its 
banks at ordinary stages of about balf a mile, and the depth of 
water in the pools at low stage is from fifteen to twenty feet, 
bed being of sand, with no rock in place ; nor is there any 
rock in the right-bank bluffs till just below Montrose, where it 
ins also in the river-bed. On the left bank or Illinois side 
the bluffs are of rock, the same as at the rapids, where they 
ave been cat through by the river. This cut begins at Mon- 
trose, where the river makes a turn at a right angle to the east- 
ward, but the depth at low water is only two to three feet, and 
he width at ordinary stages has widened to one mile. On both 
sides of the rapids the rock bluffs rise almost immediately from 
the water. The river on the rapids continues easterly about 
two miles, then turns southward and maintains this direction 
until all the rapids are passed and for a mile beyond; then 
. 
