SUEFACE FEATUEES. 45 



State of Missouri), both rise and run for the first twenty-five 

 or thirty miles of their courses upon the Drift Deposit alone. 

 The first strata that are exposed by the deepening valleys of 

 both these streams belong to the Upper coal-measures, and 

 they both continue upon the same formation until they make 

 their exit from the State, near the boundary of which they 

 have passed nearly or quite through the whole thickness of 

 that formation down to the top of the Middle coal-measures. 

 Therefore, as might be expected, both these streams are 

 very similar in their general characters so far as their Iowa 

 portions are concerned. Their valleys are usually pretty 

 well defined; but sometimes the surrounding high land slopes 

 for a mile or more gently towards the stream. They grad- 

 ually deepen from their upper portions downward, so that 

 within fifteen or twenty miles, they have reached a depth of 

 near a hundred and fifty feet below the general level of the 

 adjacent high land, which depth they retain with little in- 

 crease until they pass beyond the limits of the State, because 

 the general slope of the country is nearly concurrent with the 

 slope of the streams. 



The strata of' the Upper coal-measures consist, in this part 

 of the State, of beds of limestone alternating with those of 

 clayey and shaly composition. The latter readily soften 

 and disintegrate upon exposure to atmospheric influence and 

 the action of the streams. Thus when the rivers have cut 

 their valleys down through the series of limestone strata, 

 they reach those of clayey composition before mentioned. 

 Upon these they widen their valleys and make for themselves 

 broad flood-plains which become conspicuous features in 

 their scenery; the soil of which, owing to its origin, is stiff 

 and clayey, except where it is modified by sandy washings. 

 These broad bottom lands are particularly noticeable in the 

 southern parts of both Appanoose and Decatur counties. 



A considerable breadth of woodland occupies the bottoms 

 and valley sides along a great part of their length, but their 

 upper branches and tributaries are mostly prairie streams. 



Platte fixer. This is also a river belonging mainly to 



