334 DRAKE— THE GEOLOGY OF IXDIAX TEP.RITORY. [Sept. :^, 



part of the field the escarpments and undulating topographic fea- 

 tures are due to the varying resistant power of gently westward 

 dipping beds. 



Ouachita Mountain Structure.— In the central and northern part 

 of the Ouachita area the mountains and mesas are practically all 

 synclinal in structure, and the deeper and broader synclines form 

 the larger mountains, such as Sugar Loaf, Poteau, Cavaniol and 

 San Bois mountains; the mesas, such as the Seven Devils and 

 McChar mountains, occupy the smaller synclinal folds. In the 

 southeastern part of the field all the mountains and ridges are up- 

 turned edges of extensive hard rock beds. There is every grada- 

 tion from the sharp ridge with equal slopes on either side and its 

 steeply dipping strata, to the escarpment or bench topography with 

 its gently dipping beds. 



Ouachita Valley Structure. — In the central and northern part of 

 the Ouachita area most of the larger valleys lie along anticlinal 

 axes, while in the southeastern part of the field the valleys are in 

 the softer strata or along faults, and are nearly always parallel to the 

 structural lines. 



Folds and Faults. — The folds and faults belong principally to 

 that part of the field included in the Ouachita mountain system. 

 The limits of the folded area are not sharply marked, however, and 

 there are isolated folds and faults through and around the Ozark 

 mountains. There are two distinct systems of folds, a primary 

 system and a secondary one. The axes of the most characteristic 

 or primary system run about S. 80° W. The axes of the secondary 

 system run approximately northeast-southwest, but the different 

 folds of this s) stem vary much in direction on either side of the 

 general direction. It does not appear that these two systems of folds 

 are of different ages, because the topographic features have de- 

 veloped equally along corresponding kinds of folds regardless of the 

 direction of their axes, and also because the two systems are inti- 

 mately related, as is shown by certain folds lying in both systems. 

 As already stated, the axes of the principal folds run nearly east and 

 west and the folds are quite regular. The distinctness of the 

 secondary folding is shown by the general southwest direction of 

 many axes and by the west-northwest dip of all the rock beds in 

 the central and western part of the whole area of the reconnaissance. 

 These western beds dip nearly northwest in the southern part of the 

 area, to the west-northwest in the central, and to the west in tlie 



