372 KEYES — GRANITES AND TORPHYRIES OF THE EASTERN OZARKS. 



tioiial surface — the Tertiaiy peneplain — and the second is a moderately 

 dissected plain lying at a somewhat lower level. 



TER TIA R Y PENEPLA IN. 



The most notable physiographic feature of Missouri is the great Ter- 

 tiary peneplain which forms the general surface of the Ozark uplift. All 

 of the present topographic expression of the southern part of the State, 

 varied as it is, is but the effect of erosion in the effort to again reduce the 

 region to baselevel. In no part of the U])lift, unless it be in the Saint 

 Francois district, does an}'- portion of the pre-Tertiary surface project 

 above the broad constructional plain, and even here the nearly uniform 

 height to which the numberless peaks rise would indicate that they also 

 were practically obliterated in Tertiary time, at least as prominent sur- 

 fiice features. At the extreme end of the uplift, where the crest begins 

 to pitch to the eastward and within the boundaries of what is called the 

 ]\Iine la Motte district, the remnants of the peneplain which constitutes 

 the upland are found only in the southern or southwestern part and in 

 the northeastern portion. The latter is separated from the southwestern 

 upland by a broad expanse of lowland. The areas of upland possess 

 rugged relief, particularly in the southwest. The northeastern district 

 forms a part of tlie drainage divide which separates tlie waters which flow 

 directly into the Mississippi river from those which flow in tlie opposite 

 direction into the Saint Francois. The foundations of the upland are 

 the hardest rocks — in the southwest the porphyries and in the other parts 

 chert}' limestones. 



The region occupied by the former is deeply cut by the streams, the 

 principal one being the Saint Francois river, which traverses the area in 

 a general north-to-south direction. The tributaries are small, numerous, 

 torrential. The valleys are contracted and steep-sided, and the flood 

 plains very narrow. Above them the porphyry hills rise in high, regu- 

 larly rounded mounds with almost ])recii)itous slo})es. 



The persistent strength of the other ])ortion of the upland surface in 

 the northeast is a hard, very chert}'^ limestone which overlies softer, less 

 silicious beds of limerock. Beyond the limits of the district the ui)land 

 is continuous, and the portions appearing within the boundaries are long 

 lobes or extensions which project out from main body. These ridge-like 

 elevations terminate abrui)tl3^ in steep slopes which form sections of a 

 rather pronounced escarpment of tortuous course, but having a general 

 trend of northwest and southeast. The sides of the declivity present 

 sharply incised or crenulated outlines. The hills formed are thickly 

 covered by the hard chert}' fragments which obscure or totally hide the 

 strata underlying. Several prominent chert- covered hills which occupy 



