GENESIS OF THE ARKANSAS VALLEY 487 



In comparing the two districts, it is their differences and not 

 their points of resemblance which are most conspicuous. In 

 the Ouachita region the surface relief is notably mountainous, 

 long ridges and isolated peaks, with wide, flat-bottomed valleys 

 intervening. In the northern district the country is far from 

 appearing mountainous ; it is, for the most part, a vast undulat- 

 ing plain, but sharply and deeply dissected around the borders, 

 with the streams flowing in v-shaped valleys. In the south the 

 rocks are more or less indurated or metamorphosed, and cut at 

 intervals by eruptives. Nowhere in the north do the strata 



Ouachita Mts Boston Mts Oz.ark PuATe^u 



Fig. I. — Peneplains of the Ozark region. 



show alteration or evidence of the presence of eruptives. The 

 southern district is folded to a marked degree, approaching 

 closely the Appalachian structure ; while the northern region is 

 only gently bowed. Regarding the geological ages of the two 

 districts, the Ouachita has been thought to have been upraised 

 towards the close of the Carboniferous ; the northern area has 

 been commonly considered as having been an elevated region 

 ever since pre-Cambrian times. The present uplift, however, is 

 now believed to be of very recent origin ; and the upward 

 movement is thought to be still in progress. 



The physiographical history of the region and the relations 

 of the graded surfaces of the Ozark highlands are best indicated 

 in diagram (Fig. i). 



Two distinct base levels are discernible in the region. They 

 have been called the Cretaceous and the Tertiary peneplains. 

 These titles will be retained for the present. The first of the 

 peneplains rises out of the level savannas of the Mississippi 

 embayment, but soon becomes deeply broken as it rises and 

 passes into the Ouachita region. It is there believed to be 



