SURFACE FEATURES. 59 



In the northern part of Webster 'county the character of the 

 main valley is slightly modified by the presence there of 

 ledges and low cliffs of the snb-carboniferons limestone and 

 even by the gypsum; but the Drift Deposit is so thick that the 

 general contour of the valley is such as might be expected 

 from the prevalence there of this incoherent material. At 

 Fort Dodge the valley has reached a depth of one hundred 

 and seventy feet from the adjacent general prairie level. 

 From Fort Dodge below, it begins to assume more of the 

 usual characters of a river valley, having its winding course 

 through a well marked flood-plain. t From a point a little 

 below Fort Dodge to near Amsterdam, in Marion county, the 

 river runs all the way through and upon the Lower coal- 

 measure strata. These strata, all being friable and easily 

 disintegrated, they do not produce strongly marked features 

 in the valley. For the same reason, and also in consequence 

 of the usual heavy deposit of drift, natural exposures of 

 those strata seldom appear. Along this part of its course 

 the valley has generally irregularly sloping sides, sometimes 

 abrupt, but usually blending with the uplands in the 

 distance. The flood-plain varies from an eighth to half a 

 mile or more in width. From Amsterdam to Ottumwa the 

 sub-carboniferous limestone appears at intervals in the 

 valley-sides again, forming low cliffs occasionally ; but the 

 exposures are generally small and distant from each other. 

 Near Ottumwa the sub-carboniferous rocks pass beneath the 

 river again, bringing down the coal-measure strata into its 

 bed, but they rise again from it in the extreme northwestern 

 corner of Yan Buren county, and sub-carboniferous strata 

 resume and keep their place along the valley from there to 

 the mouth of the river. From Fort Dodge to the western 

 part of Lee county, the strata of the Lower coal-measures are 

 present in the valley, either at its bottom or resting upon the 

 sub-carboniferous strata and forming the upper portion of the 

 valley-sides. The drift is everywhere present, covering all 

 the strata, except where it has been removed by erosion, but in 

 the valley it is in a more or less modified form. 



