488 CHARLES R. KEYES 



continued northward in the mountain summits, which are often flat- 

 topped/ The later peneplain is thought to be represented in 

 the intermontane flats which are about 1500 feet lower than tops 

 of the mountains. The floor of the Arkansas valley is coinci- 

 dent with the Tertiary peneplain. Beyond the stream northward 

 the Tertiary surface rises rapidly according to Hershey,^ and 

 soon in the region of the Boston Mountains the two peneplains 

 practically merge. In Missouri only the Tertiary plain has 

 been distinguished, and this is regarded as forming the general 

 upland surface of the uplift. 



It is probable that north of the Boston Mountains it will be 

 exceedingly difficult to differentiate at any point the two pene- 

 plains. Present evidence goes to show that during the interval 

 between the formation of the two peneplains in the south the 

 erosion in the north was comparatively slight, and resulted in 

 merely lowering the general surface of the plain already formed 

 during the Cretaceous, 



Some time ago it was incidentally stated that the Ozark 

 highlands formed a single unit bowed up from the Red River to 

 the Missouri. 3 The most obvious support for this conclusion is 

 found in the physiographic development of the region. But the 

 evidence is not alone from this source. 



The physiographic data would indicate that in the Ozark 

 region the uprising since Cretaceous times has been not only 

 periodical in its character, but that it has been also differential. 

 Latel}^ the movement has been more marked in the north than 

 in the south. 



But there were special conditions existing that enabled the 

 Arkansas River to hold its own against the great barrier which 

 started to rise across its course. In a limited belt in this part of 

 the Ozark region an enormous mass of non-resistant clay shales 

 had been deposited in Carboniferous times. The thickness 

 attained was much greater than that of the Carboniferous 



'Arkansas Geol. Surv., Ann. Rept., 1890, Vol. III. 

 ^American Geologist, Vol. XXVII, p. 25, 1901. 

 3 Missouri Geol. Surv., Vol. VIII, p. 331, 1895. 



