54 PHYSICAL GEOGKAPHY. 



above the northwest corner of the State, the stream meets 

 with remarkable obstructions from the presence there of the 

 Sioux Quartzite. This formation outcrops directly across 

 the stream at that point, and causes a fall of about sixty 

 feet within a distance of half a mile, producing a series of 

 cascades and some fine examples of wild and romantic 

 scenery, quite in contrast with that of the valley below and 

 of the surrounding monotonous prairies. 



From the northwest corner of the State downward, the 

 river slope is comparatively gentle and similar to that of the 

 other streams of western Iowa. For the first twenty-five or 

 thirty miles above its mouth, the valley of the Big Sioux 

 is very broad, and has a broad, flat flood-plain, which is 

 in all respects like, and indeed continuous with, that of the 

 Missouri river. Above the point named the character of 

 the flood-plain changes, and like those of the drift-valleys, 

 it becomes usually undulatory with very gentle slopes from 

 the base of the retreating valley-sides to the stream, 

 occasionally showing indistinctly defined terraces. The 

 material of these terraces and valley bottoms — they are not 

 here true flood-plains — is principally drift, slightly altered, 

 and not alluvium properly so-called. They constitute some 

 of the finest agricultural lands of the region, being quite 

 above the reach of the highest floods. 



On our "side of the valley, however, the upland presents 

 abrupt bluffs, as steep as the incoherent materials of which 

 they are composed will stand. They are from one hundred 

 to nearly two hundred feet high above the stream, while the 

 Dakota side of the valley slopes away gently, its border 

 gradually blending with the prairie surface be3^ond. The 

 abrupt bluffs are almost continuous upon the Iowa side, 

 while the flood-plain and sloping valley-side are almost as 

 continuous on the other. Occasionally, however, along the 

 borders of Sioux and Lyon counties, the abrupt bluffs appear 

 on both sides of the valley, which is there quite narrow. 

 These bluffs are frequently scored upon their faces with deep 

 gorges, each with a small rill at its bottom. These gorges 



