106 IOWA STUDIES IN NATURAL HISTORY 



following uplift. The difficulty in distinguishing intrenched 

 meanders from curves made in other ways not involving 

 more than one cycle of erosion is brought out in Part I, 

 as are also other limitations and possibilities in the applica- 

 tion of intrenched meanders to interpretations of the ero- 

 sional histories of regions. 



The two major streams of the Driftless Area, the Mis- 

 sissippi and Wisconsin rivers, show no intrenched mean- 

 ders, in spite of the fact that they seem to have had a 

 history which would have developed such features. Both 

 streams have flood plains several times the widths of the 

 rivers and the details of their present courses are what 

 would be expected under these conditions ; but their curves, 

 are not intrenched. Except for their details, the general 

 courses of these rivers are quite remarkably straight. If 

 the streams were at grade on the Dodgeville plain and de- 

 veloped meanders there they must have straightened their 

 courses as they cut down toward the Lancaster plain. 

 Either the Lancaster peneplain was not sufficiently flat- 

 tened for the development of conspicuous meanders or 

 meanders were formed in this second cycle and cut off again 

 after uplift of the Lancaster plain. It is not unreasonable 

 to suppose that such streams would straighten themselves 

 after rejuvenation; indeed, it is not clear that a stream 

 could maintain the meanders developed in one cycle much 

 past maturity of the following cycle. Therefore, the absence 

 of intrenched meanders in these major streams is not 

 thought to argue strongly against the idea that the Drift- 

 less Area has suffered more than one cycle of erosion. On 

 the other hand the curves of these streams have nothing to 

 offer in favor of this idea. 



Conditions are somewhat different in the cases of smaller 

 streams. Upper Iowa, Yellow, Turkey, and little Maquoketa 

 rivers in Iowa are quite crooked and spurs from the valley 

 walls project into the curves. But a large part of the 

 courses of these streams lie in an area which has been 

 glaciated, and the curves may have been developed on the 

 surface of the drift and superimposed on the bedrock. But 



