256 ASHLEY — GEOLOGY OF ARKANSAS. [May 13, 



Let Fig. 2 be a very generalized columnar section, in which (a) 

 represents the Silurian novaculites, {I?) the sandstone immediately 

 overlying the novaculites, {c) the variable thickness of sandstones 

 and shales, {d) massive sandstones, and (<?) the great thickness of 

 soft and hard sandstones forming most of the section. 



The topography developed upon these beds will depend to a 

 great extent upon the character of the folds. In the case of a sim- 

 ple fold, such as are shown on Figs. 3 and 4, where erosion has 

 eaten down until the hard sandstones are exposed, an anticlinal 

 ridge will follow the axis with a valley on either side. Figs. 3 and 

 4 show the effect of the dip upon the width and character of the 

 valleys. As a rule it is found that, of the ridges formed by the 

 sandstones overlying the shales, the one on the side from which the 

 principal drainage comes is cut down so as not to present an abrupt 

 face. Fig. 5, shows the effect of two such anticlines close together. 



Generally one side of the anticline is steeper than the other, as 

 is suggested in Fig. 8, when this becomes overturned the effect is 

 modified slightly, as is shown in Fig. 6. 



Where shale underlies the sandstone of the anticlinal axis, in 

 time this anticline becomes breached (Fig. 7), and the topography 

 resembles that of which the Wall mountain (Figs. 27 and 28) is an 

 excellent example. 



If erosion has not cut down to the underlying sandstone, the 

 result will be the same, except that the anticlinal ridge is omitted 

 and the valley will be narrower, possibly very much narrower. 

 Such an anticline (Fig. 8), is difficult to locate accurately. This 

 is a common type; it is illustrated by the Prairie Bayou anticline 

 (Fig. 16). 



Overturns of the kind shown in Fig. 9, are very difficult to 

 recognize ; the mere presence of the shaly layers is frequently 

 the only suggestion of their presence. If the shaly layers were as 

 surely located and as completely isolated as in the general section, 

 Fig. 2, this could be relied upon ; but, as shown by the columnar 

 sections of PL I, it cannot be taken alone as a sure indication. 



When the shales of the section are somewhat deeper seated they 

 may not be brought within range of erosion ; then the topography 

 gives no clue to the structure. These conditions are illustrated by 

 Fig. 10. 



If an overturn occurs under such conditions as those suggested 

 in Fig. II, unless an exposure is found at the axis, it will generally 

 pass unnoticed. 



