l^'.-7.] DRAKE— THE GEOLOGY OF IXDIAX TERRITORY. o33 



tains, through Mississippi, Louisiana and central Texas. ^ There are 

 two principal types of mountains in the Ouachita system ; one of 

 these consists of long ridges, the other is the bench-and-bluff and 

 flat-topped mountain type. The mountains, ridges and valleys of 

 this region trend almost east and west, corresponding to the folds. 



The Canadian river and the Arkansas river from the mouth of 

 the Canadian river to Arkansas, practically form the northern 

 boundary of this group. For some fifteen to twenty miles south of 

 these rivers low isolated buttes, table-lands and ridges such as 

 Long mountain, the Seven Devils, McChar mountains and Back- 

 bone mountain are the outliers to the typical part of the mountain 

 system. Immediately south of these outliers are the Sugar Loaf, 

 the Cavaniol and the Sans Bois mountains, each rising nearly 3000 

 feet above tide-level. Tliey all trend about S. 80° -W., but are com- 

 pletely separated. 



These three mountains, together with the Poteau mountains, and 

 many smaller elevations, are of the bench-and-bluff type. The 

 mountains farther to the south, the Walker, the Blue, the Winding 

 Stair, the Black Fork and the Rich mountains are of the ridge type. 



Prairie Plains^- — The third topographic division comprises all 

 the country north of the Canadian river and west of Grand river. 

 The predominant topographic feature of this division is a broad 

 southeastward sloping plain, broken by gentle irregular undulations 

 and by long lines of escarpments facing east or east -southeast. Of 

 these features the east-facing escarpments are the most conspicuous. 

 They are usually but fifty to one hundred feet high, although often 

 from ten to fifteen miles long, and even much longer if local breaks 

 are not taken into account. Occasionally gentle westward slopes 

 extend from the top of one escarpment back to the base of the one 

 next west of it, but usually the inclination is not sufficient to bring 

 the base of the western escarpment down to the level of the eastern 

 one. 



Structure of the TopograpJiic Groups. — In each of these topo- 

 graphic groups the geologic structure accounts for the topographic 

 forms. The plateau type in the northeast has its strata almost 

 horizontal ; the folded type in the southern part of the field has its 

 strata irregularly folded along nearly parallel lines ; in the western 



^ Proc. Bost. Soc. Nat, Hist., Vol. xxvi, p. 477/ Am. Joicr. Sci., November, 

 i897» PP- 357-371- 

 ■■^ J. W. Powell, National Geogr. Monographs, Vol. I, No, 3, p. S3. 



