696 A. H. PURDUE 



gradually fall off to the east, sinking below the Tertiary deposits 

 just west of the St. Louis, Iron Mountain and Southern railway 

 and south of White River; also from this region of highest 

 elevation, they fall off westward to the Grand River in Indian 

 Territory/ It will be seen that the east-west line along the 

 crest of these mountains forms a gentle arch in the middle. 

 Structurally, in the western part of Arkansas, these mountains 

 are a broad, flat anticline, the strike of which is east and west. 

 According to the geologists of the Arkansas Geological Survey, 

 it appears that the extreme eastern part of the region is mono- 

 clinal in structure, with the dip to the south. ^ 



With the exception of the Illinois River in the western part 

 of the state, the drainage of the region is northward and east- 

 ward into White River, and southward into the Arkansas. The 

 direction of the streams has been determined by the slopes 

 incident to the uplift, modified in some cases by faulting and 

 flexuring. The effect of the latter upon Little Red River and 

 neighboring streams has already been noted by Professors New- 

 som and Branner.3 The westward course of the Mulberry River 

 has been determined by a fault. Detailed work of the region 

 would doubtless disclose numerous other similar examples. 



The drainage of the region is that intermediate between 

 youth and maturity. The streams are vigorous, and have com- 

 pletely dissected the plateau by the formation of gorges from 

 500 to 1000 feet deep, thus producing a very rugged topog- 

 raphy over the whole region. Between these gorges the slopes 

 often meet, forming more or less rounded hills ; but more fre- 

 quently the intervening area is occupied by flat-topped, sand- 

 stone capped hills of limited extent. 



The tributaries of both the Arkansas and the White rivers 

 have worked their way back to, and in many cases, far beyond 



' Dr. N. F. Drake, in Proc. of the Am. Phil. Soc, Vol. XXXVI, No. 156, p. 332. 

 ^ Newsom and Branner, " The Red River and Clinton monoclines, Arkansas," 

 A7ti. Geologist, Vol. XX, July 1897, pp. 1-13. 



R. A. F. Penrose, Jr., Ark. Geolog. Surv., Vol. I, 1890; section with pocket map. 

 3 Loc. cit. 



