GEOLOGY OF SOUTH WESTEKN IOWA. 299 



labor. When reclaimed and protected, these lands will rank 

 among the most durable and productive in the State. 



The whole valley of the Missouri river, so far as it forms 

 the western border of Iowa, is approximately a flat plain 

 from the borders, of which the bluffs rise abruptly on either 

 side to the height of from one hundred and fifty to near three 

 hundred feet. It has the same average slope to the south- 

 ward that the river has. The greatest inequalities of its 

 surface have been occasioned by the action of the river 

 currents at high water, and amount at most to only a few 

 feet variation from the general level of the plain. The process 

 of erosion of the valley down to its present level has been 

 continuous from the earliest part of the Terrace epoch, during 

 which time the channel has repeatedly occupied every part of 

 its valley by vibrating from side to side, gradually occupying 

 lower and lower levels also, as the valley deepened. The 

 greater part of the depth of the valley along the border of 

 Iowa has been cut through the Bluff Deposit alone, which has 

 been before described, and as the valley was deepened, bluffs 

 of this ancient deposit of the same muddy river were left 

 bordering the subsequently made valley upon either side. 



The depth to which the erosion by the river current has 

 extended, is not alone that from the general level of the 

 uplands to the level of the flood- plain; but its channel has, in 

 many places at least, been cut much deeper even than the 

 usual average depth of the water in the channel. This is 

 well shown to be the case wherever artificial excavations have 

 been made in the flood-plain, in which all the material 

 reached is readily recognized as alluvial. It is thought that 

 the strong current of the river has caused these deep excava- 

 tions in some places, and that they have been refilled by the 

 shifting of its channel. It does not seem necessary to infer 

 from these phenomena that the excavations were made by the 

 current when the land occupied a higher level, and that the 

 refilling resulted from a subsidence afterward. Evidences of 

 similar erosion beneath the usual depth of the channel of all 

 our smaller rivers and even of the creeks are not uncommon. 



