258 ASHLEY — GEOLOGY OF ARKANSAS. [May 13, 



In each of these cases, while erosion is the apparent agency, the 

 real determining factor is the comparative elevations of the original 

 surfaces as compared with the average elevations of the surround- 

 ing region. 



The anticlines may vary in height at different points, however, 

 and a single anticline may represent almost every feature figured ; 

 for, as it begins low and gradually rises, it may expose at first only 

 the upper sandstones, then the shales, then anticlinal ridges due to 

 sandstones in or under the shales, as shown diagrammatically in 

 Fig. 12. The anticlines which near the end may have low dips on 

 either side, may be overturned near the centre or for much of its 

 course. 



Fig. 12. — Diagram of topography at the end of an anticline. 



Under the circumstances represented in Fig. 12 it would seem an 

 easy matter to locate and accurately define the anticlines and syn- 

 clines. As a matter of fact, almost no such evident structural 

 topography is met with in the region under consideration. 



While the topography is largely concordant with the structure, 

 many disturbing elements enter which vitiate any conclusions that 

 may be based upon topography alone. On account of causes to 

 be spoken of later, the main drainage systems of the area are only 

 slightly controlled by structure, their general trend being oblique 

 or transverse to the strike of the folds. This results in a double 

 series of valleys cutting each other at all angles ; and through the 

 same causes the main divides are also transverse to the structure and 

 on these for a width of from half a mile to several miles the 

 topography is usually not well enough marked to indicate struc- 

 ture. Many other local and minor causes, such as change in the 

 character of the rocks, faulting, crushing of strata, constant varia- 

 tion in the character of the folds, and others enter as factors to 

 complicate the topography. Though it is probable that the topog- 

 raphy, if worked out in detail, would be found to be closely 

 governed by the structure, modified by the factors mentioned, 



