PRIMARY AND SECONDARY VALLEYS. 239 



across the stratification. In a region of great rock flexures the larger val- 

 leys are generally of this type. This area is no exception to the rule. The 

 Hartford valley, in Sebastian county, is a remnant of a grand anticlinal 

 valley ; the Poteau mountain range forms its southern boundary, but of 

 the northern border ridge only a fragment is left in Sugar Loaf mountain. 

 The valley north of Sugar Loaf mountain is a similar remnant. Such val- 

 leys as these, however, are what may be called primary valleys, their sur- 

 faces being broken by ridge lines, between which lie secondary valleys of 

 monoclinal or anticlinal form. The terminal anticlinal valleys resulting 

 from the fusion of two such secondary monoclinal depressions have already 

 been referred to. Examples of purely anticlinal shape are Coops prairie, 

 surrounded by Coops ridge ; and Washburn valley, occupying the center of 

 the anticlinal arch of the same name. Such valleys are, however, rare here, 

 and, as compared with the number of monoclinal valleys, the same may be 

 said of all regions, including even those where numerous folds of the strata 

 are developed ; for each such fold can exhibit only one anticlinal valley 

 which will be centrally located along the axis of flexure, whereas each dis- 

 tinct and persistent stratum of hard material affected by such flexure will be 

 developed by erosion into a dividing ridge between two monoclinal valleys. 



Synclinal Valleys. — These are both topographic and strati graphic basins, 

 formed, generally, by convergently dipping strata of monoclinal ridges. 

 Complete and unbroken valleys of this type are exceedingly rare. The 

 valley in the eastern extension of Sugar Loaf mountain is of this type, as is 

 also that which extends from Greenwood westward. Other illustrations are 

 the Potato hill prairie, the Philpott coal basin, and the Ouita coal basin. 



Valleys in horizontal Strata. — These valleys differ from each of the three 

 classes heretofore described in the extreme irregularity of their outline. No 

 normal plan for such depressions exists. The typical cross-section is that of 

 a flat plane bounded on both sides by abrupt escarpments, i. e., is similar to 

 the cross-section of an anticlinal valley. When of oblong shape they follow 

 no prevalent trend ; their directions are adventitious. Laterally, resistance 

 to erosion is equal in all directions, and hence their growth is in all direc- 

 tions at a rate dependent entirely upon external conditions. The canon, the 

 narrow gorge, is the rudimentary form of a valley in horizontal strata. The 

 intense corrasive energies of degradation are first spent before the slower 

 processes of erosion and sapping show their effects in lateral degradation. 



A gorge of this type is that of James' fork, south of Hackett City. Mas- 

 sard prairie, south of Fort Smith, may be considered an example of a valley 

 eroded out of horizontal strata. Slight undulations in the rocks exist here, 

 but are not sufficient to lend form to the valley. The characteristic irregu- 

 larity of outline, as defined by the bounding ridges on the north and south, 

 will be particularly noticed. It is an old valley ; corrasion is no longer 



