74 PHYSICAL GEOGRAPHY. 



miles. Its beach is clean and gravelly; and nothing is lacking 

 to make it a beautiful and interesting region but the presence 

 of forest trees, for the region all around is an open prairie. 

 Trees mil grow so rapidly when planted and protected from 

 the fires, that this defect will, in a few years, be remedied. 



The outlets of all these drift-lakes, except that of Okoboji, 

 are dry during a portion of the year, when they appear to 

 have no outlet, as they are usually shallow and small, and 

 frequently covered with a growth of coarse grass and sedge. 



In the wet portion of the year the water escapes through 

 these, but at other times the escape is evidently through the 

 gravelly drift subsoil. 



9. WALLED LAKES. 



The lakes thus far mentioned are the largest in the State; 

 but along the watersheds of northern Iowa great numbers of 

 smaller ones exist, varying from half a mile to two or three 

 miles in diameter. They are usually pleasant, shallow sheets 

 of water, occupying the depressions among the undulations 

 of the surface. One of these small lakes in Wright county, 

 and another in Sac county have each received the name of 

 "Walled Lake" on account of the existence of embankments 

 upon their borders which are popularly believed to have been 

 the work of ancient aboriginal inhabitants, and concerning 

 which many fanciful stories have been circulated in the public 

 prints. These two lakes are no more worthy of such dis- 

 tinction than any others of their class, because all the drift 

 lakes that have been examined, and this comprises nearly all 

 that exist in the State, not one of them has been found desti- 

 tute of the so-called walls along some portion of its shore; 

 and some of them even exceed in interest the two that have 

 attracted so much attention. 



A true description of these popularly-called walls, but which 

 I shall term embankments, will be best understood if given in 

 connection with an account of their origin. 



When a pile of sand from the river shore has been left hy 

 the workmen for a long time exposed to the rains, the gravel 



