STRUCTURE REFLECTED IX RIDGE SYSTEMS. 287 



outline, of which arch the dome has beeu eroded and the underlying shales 

 eaten out, so that now only a monoclinal rim is left. The height of the 

 ridge varies, and the crest line is broken in places into a succession of small 

 peaks ; the greatest altitudes are at the termini of the anticlinal curve. 



The systent of ridges immediately north of Coops ridge is particularly 

 well-marked ; the rocks here dip northward at an angle of about 20°, but 

 eastward this dip flattens and the ridges terminate in typical canoe-shaped 

 points. 



Jennings hill and the ridges which surround it in concentric ellipses are 

 beautiful developments from a synclinal structure. These ridges are all 

 monoclinal, and are composed of sandstones, underlain by shales dipping 5° 

 to 10° convergently toward the common center line of the series of ellipses. 



Of chief prominence among the ridges are those which flank the Wash- 

 burn valley, on each side of the anticlinal axis. The inverted canoe-shaped 

 connection of the two systems at the western end is boldly brought out both 

 by the elevation of the ridges at this poiut and by their sharply defined out- 

 lines. Toward the east, some of the ridges of marked prominence die out 

 or become relatively insignificant, an undoubted indication of a concomitant 

 thinning of the hard layers, to which these ridges owe their prominence 

 elsewhere. 



A similar system of ridges is that of which the eastern termination is at 

 Greenwood. It follows in the same way a strong anticlinal flexure ; but 

 on account of the faulting which accompanied Ais flexing (see figure 4, page 

 229) the ridges north of the axis are not always of prominence. 



Many other instances of similar ridges might be cited and their local 

 characteristics referred to, but such detail would be out of place here. 

 Enough has beeu said to direct attention to the existence and character of 

 these features of relief; their distribution aud special characters can be 

 studied on the map. 



North of the Arkansas, between it and the Boston mountains, the char- 

 acter of much of the topography is essentially diflTerent from that last de- 

 scribed, and from that south of the river generally : the hills are closely 

 crowded, their outlines are very irregular, they have no uniformity in trend, 

 and the slopes are gentle or steep, according to the meanderings of a ca- 

 pricious drainage system. The shales and sandstones, so common elsewhere, 

 make up these hills, and they are generally capped by beds of the latter. 

 As may be inferred, no pronounced and persistent dip of the strata character- 

 izes this region. The angle of dip seldom exceeds 5°, and the direction is 

 fluctuating. In short, the topography here is typically such as results from 

 erosion in heterogeneous strata which lie in a horizontal position. The 

 region north of the river has been one of repose, and erosion dominates 

 stratigraphy ; that south of the river has been one of disturbance, and 

 stratigraphy asserts itself in the topography. 



