GEOLOGY OF SOUTHWESTERN IOWA. 301 



form of longitudinal ridges and parallel depressions, having 

 the same general direction which the currents had that 

 caused them. These ridges are not such in the strict sense of 

 the word, but are comparatively broad, flat spaces between 

 the shallow depressions. Some of the latter even now serve 

 as auxiliary channels for carrying off the water in times of 

 flood, a part of which remain as shallow ponds at other 

 times, but the majority become dry as soon as the floods are 

 gone. None of these depressions are really parts of ancient 

 beds of the main channel, and now remain quite beyond the 

 reach of the highest floods, being filled at all times with 

 comparatively pure water, and stocked with fish. Such a one 

 exists at the foot of the bluffs, just above the point where 

 Soldier river enters the flood-plain. This one we sounded in 

 company with Mr. Solomon J. Smith, the owner of the land 

 upon which it is located, and found it at one point to 

 measure more than thirty feet deep, which is estimated to be 

 about fifteen feet deeper than the surface of the Missouri river 

 at low water, the channel of which is now eight miles away to 

 the westward. As one stands upon the adjacent bluffs he is 

 able to trace by the eye great numbers of these longitudinal 

 depressions that are too shallow and indistinct to attract 

 attention while he is passing over them. These, together 

 with those previously mentioned, are all evidences, if farther 

 evidence were necessary, of the former occupancy of every 

 part of the plain by the river, but it must be remembered that 

 the greater part of that portion of the flood-plain which lies 

 in Iowa is not now reached by the highest floods of the great 

 river. * 



It should be remarked that some of the longitudinal 

 depressions before referred to, have been caused, not by the 

 main or accessory channels of the Missouri river itself, but 

 by those of such of its tributaries as enter its flood-plain on 



* The term, flood-plain, as here used, is a common term to designate flat-lands 

 that border all rivers, and owe their origin to the action of the former floods in the 

 process of eroding their valleys. Some of such lands may be, and often are, much 

 above the reach of present floods. 



