SUBFACE FEATURES. 53 



described. It rises upon the drift, and flowing southward, it 

 enters the region of the Bluff Deposit a little north of the 

 centre of Plymouth county, and continues upon this deposit 

 from thence to its mouth near Sioux City. Along this part 

 of its course it has not, like the other streams of the region 

 of the Bluff Deposit, cut its valley down through it, so as to 

 expose the drift, except at a few points near Sioux City; 

 although it seems certain that the drift cannot be anywhere 

 very far beneath its bed. 



This river is, almost from its source to its mouth, a prairie 

 stream, with gently sloping valley -sides which blend 

 gradually with the uplands, giving the valley the appear- 

 ance of being very shallow. It reaches a depth of about 

 one hundred and fifty feet from the adjacent uplands. A 

 single slight exposure of sandstone of Cretaceous age, 

 occurs in the valley near Sioux City, and it is the only 

 known exposure of rock of any kind along its whole length. 

 Near this exposure is a mill-site, but farther up, the stream 

 is not very valuable for such purposes. 



Rock river. This stream was evidently so named from 

 the fact that considerable exposures of the red Sioux 

 Quartzite occur along the main branch of the stream in 

 Minnesota, a few miles north of our State boundary. Within 

 the State, however, both it and all its branches are drift- 

 streams; that is, do strata of any kind are exposed in their 

 valleys, and nothing but drift is to be seen in and around 

 them. They are shallow valleys, with gently sloping sides, 

 which gradually blend with the undulating prairie surface. 

 The beds and banks of the streams are usually sandy and 

 gravelly with occasional boulders intermixed. 



Big Sioux river. The valley of this river from the north- 

 west corner of the State to its mouth, possesses much the 

 same characters as all the streams of the surface deposits. 

 This results from the friable character of the strata that 

 underlie the surface along its borders, in addition to the fact 

 that the surface deposits also cover them quite thickly, 

 especially on the Iowa side. At Sioux Falls, a few miles 



