238 A. WINSLOW GEOLOGY OF WESTERN ARKANSAS. 



Valleys: Geueral Character. — The greater. part of the area treated in this 

 paper may, in a broad sense, be considered part of one great valley — that is, 

 as constituting part of a single drainage basin, bounded on the north and 

 south by mountain ranges. It is here intended, however, to treat of valleys 

 in a more restricted sense, as those depressions which intervene between and 

 are complementary to all the individual mountains, ridges, and other minor 

 elevations which have already been described. With reference to the 

 geologic structure these valleys may be classed as follows : 1. Monoclinal 

 valleys ; 2. Anticlinal valleys ; 3. Synclinal valleys ; 4. Valleys in horizontal 

 strata. With reference to surface detail, to soil, to vegetation, each and all 

 of these various classes may be, in whole or in part, either rugged, undu- 

 lating, or flat ; may be wet alluvial bottoms, or dry loamy or rocky uplands ; 

 may be densely covered with forest growth, or may be prairies. 



Valleys of the first three classes preeminently characterize areas of regular 

 flexing of heterogeneous, non-metamorphosed strata. In regions of great con- 

 tortion and rock crumpling, stratigraphic divisions have a too intricate dis- 

 tribution to control corrasion ; in homogeneous strata, degradation is general 

 and uniform, and there are no guiding planes to direct the forces of erosion 

 along the lines of least resistance ; and metamorphosed strata, even when not 

 contorted, possess in a great degree the attribute of homogeneity and offer 

 approximately equal resistance in all directions to degradation. The pre- 

 dominant rocks of western Arkansas are sandstones and shales. We have 

 seen that sandstone is the distinctive ridge-forming material. Similarly the 

 fissile, easily eroded shales underlie the valleys almost without exception. 



Monoclinal Valleys. — These are the depressions between parallel mono- 

 clinal ridges, and agree with them in trend. Examples of such valleys are 

 to be seen between the parallel ridges north of the Poteau mountains. 

 Where such coalesce at the point of an anticlinal they broaden correspond- 

 ingly and are bounded by steep walls of converging ridges, giving the val- 

 leys slipper-shaped outlines. At the terminus of a syncline a broadening 

 takes place by a similar coalescence, but the valley here differs from the last 

 in being bounded by converging ridges which present their gentle slopes 

 towards the intervening depression. 



By far the best development of monoclinal valleys, however, is between 

 the members of those prominent systems of ridges which lie immediately 

 north and south of Washburn valley. On the south side, near the eastern 

 end of the area mapped, these depressions are neither deep nor broad, by 

 reason of the thinning of the ridge-forming sandstone ; while at the western 

 end, the several valleys of the southern system sweep around in beautiful 

 parallel curves and, by reason of the compression and verticality of the strata 

 at this point, coalesce with a single valley of the northern system. 



Anticlinal Valleys. — These, like monoclinal valleys, are depressions, gen- 

 erally between monoclinal ridges; but unlike those, both bounding slopes are 



