OKLAHOMA ACADEMY OF SCIENCE 119 



ral differences as compared with adjacent areas. If any important 

 difference in resistance exists it seems to be in favor of the Canad- 

 ian basin, since in the Washita basin erosion must be checked by 

 the course White Horse sandstone extensively developed in part 

 of that region. 



(3) That the ridge has resulted from conditions outside 

 the region in question which have reduced the downward cutting 

 of the North Canadian and South Canadian rivers and incidently 

 their tributaries as compared with the downward cutting of the 

 Warhita and Ciirarrcn. 



The writer believes this to be the case. This hypothesis would 

 seem to account for the narrowness of these basins since the lower 

 elevation of the beds of the Washita and Cimarron would not only 

 produce a shifting of the divides toward the Canadians thus nar- 

 rowing these basins, but also would permit a greater dissection and 

 lowering of the surface within their basins. 



If the last hypothesis is the correct one it remains to ascertain, 

 if possible, what conditions are responsible for the comparatively 

 s'ow downward cutting by the North and South Canadian. Much 

 work must be done on all of the rivers in question before a con- 

 clusive answer can be given. However, the writer believes it to 

 be large'y due to two conditions : 



1. The fact that the South Canadian and North Canadian 

 flow across resistant rock formations in the area of Penn-ylvanian 

 rocks eastward, particularly such formations as the Semino'e con- 

 glomerate, Thurman sandstone, Senora formation, and Calvin sand- 

 stone. There is a well defined narrows some 300 feet deep in 

 the "''Torth Canadian valley in township UN, R.14E., Mcintosh 

 county, where the stream is cut':ing through sandstones* (Thur- 

 man). Aho the town named, Keokuk Falls, in Eastern Seminole 

 county sueests this, although the writer has not had an opportunity 

 to vi^it this place. Likewise, the South Canadian in the vicinity 

 of Calvin has a rather pronounced narrows. These formations 

 appear to have formed a temporary base level. A similar result 

 could have been produced by uplift but the fact that the narrows 

 are lirrited to the resistant formations favors the above interpreta- 

 tion. Since these formations grade into shales to the northwardt 

 the Cimarron-Arkansas drainage crosses the upper Pennsylvaniin 

 rocks where they are less resistant while the Washita-Red river 

 drainage goes farther south and hence misses these resistant sand- 

 stones. 



2. Downv/ard cutting by the South Canadian appears to be 



*Canadian Quadrangle U. S. G. S. 



