274 ASHLEY — GEOLOGY OF ARKANSAS. [May 13, 



Hay^ having a strike about 20° north of west. In this same strike 

 Prof. T. C. Hopkins of this survey found an anticline on Bell's 

 creek in 7 S., 21 W., section 18, northvvejst quarter, and also in 

 7 S., 22 W., section 11, southwest quarter, on Terre Noir creek. 

 This anticline having a strike of N., 7° W., runs at a high angle to 

 the general trend of the structure of this region. 



Straight Mountain Anticline. — The relation of the Straight moun- 

 tain anticline to the structure of the Chalybeate mountain to the 

 east is not known ; it is assumed as starting at Antoine creek in 

 6 S., 23 W., section ^t^, southeast quarter. It produces an anti- 

 clinal ridge, known as the Straight mountain, from two hundred to 

 three hundred feet high, running a little south of west into 6 S., 

 24 W., section 36, and from this point is continued as the Wall 

 mountain. It is broken at one point by Kirkland creek, which 

 makes a gap in it. 



Wall Mountain Anticline. — Wall mountain anticline is a continu- 

 ation of the Straight mountain fold, and runs south of west to the 

 Little Missouri river, which it reaches in 7 S., 25 W., southwest 

 quarter of section 7. Starting from the Hot Springs road in 6 S., 

 24 W., section 36, for about a mile it forms an anticlinal ridge 

 three hundred feet high*; beyond this a little stream has cut its way 

 into the anticline, and lowered it somewhat. This gives an excel- 

 lent example of a breached anticline (Figs. 27 and 28). 



In the sections given in Fig. 27, I shows the profile of the high 

 unbroken ridge in the background toward the east ; II, III and IV 

 are successive profiles coming toward the foreground (westward), 

 showing the cutting down of the centre of the ridge by the stream 

 which originally cut in, and now drains by way of the gap shown 

 in section IV ; the full line gives a section on the west side of the 

 gap. One of the interesting features of this ridge is shown in 

 the illustration. This is a great wall, at one place about sixty 

 feet high, crowning the summit of the north arm of the ridge for 

 more than half a mile. It is composed of a layer of hard sand- 

 stone eight feet thick, rising perpendicularly. The manner in which 

 it has withstood erosion is due partly to its hardiness and partly to 

 its being so exactly vertical that detached masses simply rest in their 

 places. The outcrop of the same layer of rock shows along the 

 summit of the southern arm. 



This weathering of the rocks in vertical walls is not uncommon 



^ Geol. Surv. of Ark., Rep. for 1888, Vol. ii, p. 271. 



