PLANT LIFE IN OKLAHOMA. 41 



estimated, in some instances, as little as 10 per cent of it in the soil. 

 The erosion caused by the resulting freshets and floods was great and 

 the intensely muddy water of the streams carried much sand. The day 

 and dissolved substances were mostly carried on to larger rivers, but 

 the sand was dropped in the river bed in the shape of bars as the velocity 

 of the water diminished. Long periods of dry weather often intervened 

 between rains, and in these drouth periods the rivers often became com- 

 pletely dry. During the summer dry periods the wind blows almost 

 constantly from the south and has a velocity of from 10 to 40 miles per 

 hour. The finer sand is carried by such wind and during most of the 

 time, probably, since the Tertiary period, the winds have 'been strewing 

 northward much of the sand brought down by the spring freshets. The 

 area in the State thus covered by the sand from these four rivers is esti- 

 mated to be between 3,000 and 4,000 square miles. 



The average rainfall here has not increased in the last few decades, 

 but the cultivation of the soil and growing of crops and trees since the 

 settlement of the country have resulted apparently in the rainfall occur- 

 ing more gently and for longer duration. This fact, together with the 

 receptive condition of the soil caused by cultivation, results in a much 

 larger part of the precipitation entering the soil and in fewer freshets 

 and floods as well as less erosion. The sand supply of the river beds is 

 no longer greatly increased and the sand is mostly kept moist during 

 the summer and not carried by the wind. These rivers are now seldom 

 dry and the areas of shifting sand are much reduced. In a few places 

 the sand still drifts during the summer. In the southwestern part of 

 Woods County, there are several square miles covered with shifting dunes 

 and the region is advancing northward covering elm and Cottonwood 

 trees and causing Dog Creek to change its course. 



The plant life of the sand-hills consists of fewer species than that 

 of the adjacent non-sandy regions, the number being greatest along the 

 margins of the sandy areas, in better watered places and on sand-hills 

 where sand has long since stopped drifting. The Waynoka dunes, some 

 of which are 75 feet high, have no plant life except an occasional top of 

 a buried cottonwood still projecting above the drifting surface. 



BLACK MESA REGION. 



Where the Cimarron Eiver enters the northwestern corner of the 

 State it traverses a rather deep valley or broad canyon bounded by sand- 

 stone-capped bluffs and buttes on the south and by the Black Mesa on 

 the north. The Black Mesa is a bed of lava or mal pais rock, now 

 standing above the level of neighboring elevations. The flora is distinctly 

 western. 



SOILS. 



The great variety of rock formations found in the regions above 

 named produces soils differing much in character. The granites, lime- 

 stones, sandstones, shalesi, and clays each make different types of soil. 



