368 GRASSES OF IOWA. 



normal enough as far as direction is concerned. The erosion has not 

 been, however, so vigorous as in the area of older drift, and the stream 

 action seems to have about equalled that in the northeast. In the absence 

 of lakes, but occasional presence of sloughs, the country also resembles the 

 latter. The exact relations of this area to that lying east of it are as yet 

 unknown. 



In all this northern portion the drift is even thicker than in the 

 southern half of the state. Rock exposures, except in the northeast, are 

 rare, and bluff slopes, except along the major streams, are unusual. Ex- 

 cept in i the extreme northeastern part of the state, the area is drift cov- 

 ered, and the smoother, gentler slopes which mark drift covered areas are 

 common. 



GEOLOGY. 



The physiography of the state afford an accurate index to its geol- 

 ogy, especially if the younger or surface formations be considered. So far 

 as plants are concerned it is the geology of these surface formations which 

 is especially important. Over much of the larger portion of the state the 

 underlying rocks crop out at only rare intervals, and while these occa- 

 sional outcrops afford often the peculiar conditions most favorable to cer- 

 tain individual plants, the major flora of the region must be that sup- 

 ported by the drift and related soils. The underlying strata have but a 

 secondary importance from the present point of view, and will be treated 

 but cursorily. 



THE ROCK SERIES. 



Sioux Quart zite. The oldest rock in the state is exposed over a few 

 square miles in the northwestern part of Lyon county. It is the Sioux 

 quartzite and consists of a hard, vitreous quartzite, usually thoroughly 

 indurated. It does not yield readily to weathering, except that frost 

 action breaks it into rectangular blocks, and aids in the formation of the 

 river canyons familiar in the region. (See figure 256.) The quartzite 

 contributes but little to the soil, but its influence in a physical way has 

 brought about a distinctive flora. 1 The area of outcrop is small, but the 

 quartzite and certain associated porphyries, granites and schists occa- 

 sionally found in the deep wells of the region formed probably the 

 earliest portion of Iowa to be exposed. These beds are part of a long 

 tongue stretching southwest from the Archean complex of Minne- 

 sota, and are related to the older rocks of central Wisconsin. The 



•Shimek : Proc. Iowa Acad. Sci. , 4 : 72-77. 



