60 PHYSICAL GEOGRAPHY. 



It is difficult to state any definite average depth to the 

 valley of the Des Moines, because its borders are so indis- 

 tinctly defined; but it seldom exceeds one hundred and 

 seventy -five from the adjacent uplands. Its flood-plain is 

 frequently sandy, from the debris of the sandstone and sandy 

 shales of the coal-measures, produced by their removal in 

 the process of the formation of the valley. The valley 

 almost nowhere receives strongly marked features from any 

 of its underlying strata, except perhaps, in Yan Buren 

 county, where there is an uplift that has brought up the 

 Keokuk member of the sub-carboniferous limestone. In this 

 county the valley is in some places quite narrow, the 

 uplands approaching the stream on both sides. 



The principal tributaries of the Des Moines are upon its 

 western side. These are the " Three rivers " and the Raccoon, 

 which together drain a large part of central Iowa. 



The "three rivers," namely, South, Middle, and North 

 rivers, have their rise in the region occupied by the Upper coal- 

 measure limestone formation, flow eastward over the Middle 

 coal-measures, and enter the valley of the Des Moines, upon 

 the Lower coal-measures. These streams, particularly South 

 and Middle rivers, have the sides of their valleys frequently 

 bordered by high, rocky cliffs, along those parts of their 

 courses which traverse the Upper coal-measure limestone. 



Raccoon river has its rise by three principal branches upon 

 the heavy surface deposits of the Middle region of western 

 Iowa, and along the greater part of its course it has exca- 

 vated its valley out of those deposits and the Middle 

 coal-measures alone. The strata of the last named formation 

 have yielded readily to denuding action, and consequently 

 they are not often exposed in the banks or valley-sides, being 

 covered by drift and their own debris. For this reason also, 

 they do not strongly characterize the valley, although it is a 

 well marked one, and sometimes reaches a depth of one 

 hundred and seventy-five feet from the general level of the 

 uplands. 



The valleys of the Des Moines and its branches are 



