70 PHYSICAL GEOGRAPHY. 



close of the Glacial epoch. Indeed the valleys of the upper 

 branches of every river in the State, have no greater antiquity 

 than this, for they all rise and now npon deposits no older 

 than the drift. Some of the valleys of western Iowa are 

 wholly of more recent origin than the drift. These are exca- 

 vated ont of the Bluff Deposit alone, which is of lacastral 

 origin, and, as will be shown in the next chapter, is wholly of 

 later date than the drift. 



8. LAKES. 



The largest bodies of still water in Iowa are so small that 

 they wonld receive in some parts of the conntry only the 

 designation of " ponds ;"but no distinction of this kind is 

 made by onr people, all snch bodies of water, whether large 

 or small, being called lakes. The lakes of Iowa, taken as a 

 whole, are not very conspicuons features, but they are well 

 worthy of separate consideration. They may be properly 

 divided into two distinct classes, the difference being not only 

 in their existing characteristics, but also in the mode of their 

 origin, and in the relative age of the deposits upon which 

 they rest, for although they are all of post-Tertiary age, they 

 were not simultaneonsly formed. 



The first may be called drift-lakes, having had their origin 

 in the depressions left in the surface of the drift at the close 

 of the Glacial epoch, and have rested directly upon the undis- 

 turbed surface of the Drift Deposit ever since the glaciers 

 disappeared. The others may be properly called fluvatile or 

 allumal lakes, because they have had their origin by the action 

 of rivers, while cutting ont their own valleys from the general 

 surface of the drift as it existed at the close of the Glacial 

 epoch, and are now found resting npon the alluvium as the 

 others rest upon the drift. The erosion of the valleys having 

 been accomplished by the vibration of the stream from side to 

 side of its bottom or flood-plain, alternately occupying and 

 abandoning all portions of it successively; the fluvatile lakes 

 have all been formed since the others were, and originated in 



