236 A. WINSLOW — GEOLOGY OF WESTERN ARKANSAS. 



the former in geologic interest. In height these hills vary from low mounds, 

 20 or 50 feet high^ to diminutive mountains, 400 or 500 feet above the sur- 

 rounding country. As a rule, however, the height does not exceed 200 feet, 

 and the average ridge is in the neighborhood of 150 feet high. 



The distinction between ridge and mesa implied by the a])ove heading is 

 based upon a difference in cross-section which admits of this division of the 

 minor elevations into two classes. The first class, or that of ridges, includes 

 those long, sharp-crested monoclinal backbones with no summit areas and 

 whose sides slope immediately away from the crest in both directions. Some- 

 times both slopes are steep, but generally one is precipitous and rocky while 

 the other is gentle and adapted to cultivation. The second class, or that of 

 mesas, refers more particularly to flat-topped or gently sloping and undulat- 

 ing elevations of horizontal structure, approaching the character of plateaus, 

 with steep sides partially or wholly surrounding the broad summit. This 

 distinction, based upon structural differences, is by no means a rigid one, and 

 the two classes are found to merge by insensible gradations. 



The distribution of these ridges and hills, though obedient to the laws of 

 geologic structure, is subject to so many modifications through influence of 

 stratigraphy and erosion that, without careful study, all seems chaos. By 

 reference to the map it wdll be seen, however, that what are distinguished as 

 ridges can be generally grouped into a system, the members of which run in 

 long parallel lines with intervening narrow valleys. These members are 

 frequently broken by gaps, vary considerably in height and prominence, 

 are squeezed together, appear to die out, curve around in loops, and coalesce 

 to form one flat-topped, canoe-shaped end. Throughout these changes, how- 

 ever, each ridge can be recognized in all its different phases, and the idea of 

 order is traceable throughout; while with the plateau-shaped hills the dis- 

 tribution is as irregular as the outline is varied, and the characteristics of 

 form might almost be called accidental. 



Another type of hills, commonly known as " potato hills," may properly 

 be termed cones. They are of conical shape, slope steeply on all sides, and 

 are generally made up of shale with, perhaps, a sandstone cap. They prop- 

 erly represent the last stage of erosion of the mesa. 



Immediately north of the Poteau mountains there is a system of monoclinal 

 ridges, composed of sandstone strata dipping toward the mountains ; near the 

 mountains, and especially along the larger drainage channels, they are some_ 

 times well-nigh obliterated by the combined effects of torrential flows and 

 the accumulations of mountain debris. Outside of such areas, however, 

 they are easily traced, and their topographic persistence is often remarkable. 



Coops ridge, a few miles north of the Poteau mountains, is especially 

 worthy of notice on account of its isolated position and the peculiarity of its 

 structure. It is evidently the remains of a quaquaversal arch of elliptical 



