332 DRAKE— THE GEOLOGY OF INDIAN TERRITORY. [Sept. 3, 



and the Ouachita mountain systems. The westward deflection of 

 the Illinois and Grand rivers is due to the Ozark uplift, while the 

 northern deflection of the Canadian river is caused by the uplifted 

 folds of the Ouachita mountain system. Thus the rivers approach 

 each other on either side of the Arkansas river along the border 

 lines of these topographic groups. 



The Ozark Mountain System. — This term was formerly applied 

 to the mountainous area of southern Missouri, northwestern and 

 central western Arkansas and east central Indian Territory. The 

 name is, however, properly restricted to the plateau or northern 

 part of this area, and the name, Ouachita, has been applied to the 

 folded mountains of the southern part of the area. 



The Ozark mountains in the Indian Territory are divided into 

 two distinct topographic areas. One lying east of Grand river and 

 north of Tahlequah and Evansville is the lower table-land of the 

 two, and contains flat lands and also areas of sharp narrow ridges 

 and small V-shaped canyons. The other portion of the Ozark 

 system is more elevated and is the western prolongation of the 

 Boston mountains. This area is triangular in shape ; the apex of 

 the triangle is about four miles southeast of Fort Gibson, the base lies 

 south of Evansville, and is about fifteen miles across, measured 

 along the Arkansas-Indian Territory line. This table-land is about 

 1500 feet above tide-level. It is usually bounded by steep escarp- 

 ments on the north side and by both escarpments and slopes on the 

 south. Several streams have cut through this plateau and at present 

 flow through narrow valleys or canyons. The escarpment on the 

 north side of the Boston mountains has a general east-west direc- 

 tion, but it is a very irregular line with plateau tongues and outliers 

 extending northward along the face of the mountain ; these are 

 erosion remnants showing the former extension of the lower car- 

 boniferous strata. 



The Ouachita Mountain System. — The elevated topographic 

 group in the southern part of the field is an extension of the Ouachita 

 mountain system which forms such a typical area of folded strata 

 and parallel ridges and valleys in western Arkansas. This system was 

 named ^ by Dr. J. C. Branner, who thinks it is an outlier of the Ai> 

 palachian system and the structural equivalent of the Cincinnati- 

 Nashville anticline. The Appalachian system, he thinks, once ex- 

 tended across the Mississippi Valley south of the Ouachita moun- 



^ Ann. Kept. Geol. Snrv. Afk.,\o\. ii, 18SS, p. 175. 



