38 PHYSICAL GEOGBAPHY. 



The double dotted line indicates the position of the Great 

 Watershed, while the single one indicates that of the highest 

 ridge of land between the two great rivers where that ridge 

 does not constitute the Great Watershed. 



The general surface of the State, as before stated, is gently 

 undulatory, and it is mainly the development of its drainage 

 systems that has cansed it to be so. This fact is beautifully 

 illustrated upon the prairies of southern Iowa which have a 

 better defined general level than those of other parts of the 

 State. As one stands upon those broad prairies and sweeps 

 the well-defined, ocean-like horizon with his level, he finds 

 the bubble everywhere resting upon the cross-wire, except 

 where the distant, dark line of forest foliage reveals the 

 presence of a stream; here the original level has been lost by 

 erosion. Approaching these slightly depressed regions he 

 finds the surface to become undulating, like the smooth 

 rolling of a sea; but looking closely, he will see that every 

 depression leads into a still deeper one, until the upper 

 branches of the streams are reached, the waters of which are 

 often more than a hundred and fifty feet below the prairie 

 level from which he started, and the surfaces of the larger 

 streams are sometimes near a hundred feet deeper still. 



These details of the drainage are too minute to be repre- 

 sented in the foregoing diagram, or even upon the largest 

 maps. The streams themselves are considered .under the 

 following head : 



4. RIVERS AND THEIR VALLEYS. 



Since rivers, next to mountains, constitute the most con- 

 spicuous features in the physical geography of a region, and 

 Iowa being, as already shown, quite destitute of mountains, 

 or even ranges of hills, a brief description of its rivers and 

 river systems is here introduced, for the purpose of presenting 

 as clear an idea as possible of the surface features of our 

 State. 



Its inequalities of surface are due almost alone to its 

 streams, for these, as before intimated, have eroded their own 



