SUKFACE FEATUEES. 57 



lake of its first waters ; but the only effect then was to cause 

 it to cut its valley out of the deposits also, its own muddy 

 waters had previously formed. Thus, all along the valley of 

 that river, so far as it forms the western boundary of Iowa, the 

 bluffs which border it are composed of that fine sedimentary 

 material known as the Bluff Deposit. These bluffs form a 

 distinct and abrupt border along each side of the broad, 

 level Hood-plain, the width of which varies from five to fifteen 

 miles, while the original sedimentary deposit stretches far 

 inland on both sides. 



6. RIVERS OF THE EASTERN SYSTEM. 



The eastern system of Iowa drainage is rendered more 

 complete as a system, if we include with it the drainage of 

 a part of southeastern Minnesota. By doing so we find 

 the interesting fact that the greater part of the region thus 

 drained, is entirely enclosed by three rivers and their tribu- 

 taries, which complete enclosure is caused by two of the 

 streams having a common source and flowing from it in 

 different directions. These are the East Fork of the Des 

 Moines and the Blue Earth, a tributary of the Minnesota 

 river. They rise in Crocker county, in what is called 

 Union Slough by the inhabitants. The slough is a shallow, 

 longitudinal depression in the general prairie level, having 

 a north and south direction and much the appearance 

 of the valley of an ordinary prairie creek. It is without 

 water during the greater part of the year; but in wet 

 seasons it is filled to a depth of several feet, and as each 

 end of it opens into the respective streams named, one might 

 then start there with a light boat, and choose whether he 

 should perform a water-journey to the Mississippi by way of 

 the Blue Earth and Minnesota rivers, or by way of the Des 

 Moines. At such times, a large part of the region of the 

 eastern drainage system of Iowa, together with its adjacent 

 part of Minnesota, becomes an island. 



The rivers of the eastern system have as a whole quite a 

 8 



