92 JOURNAL OF THE ARNOLD ARBORETUM [vol. n 



conditions is augmented by the frequent tornados, blizzards and long peri- 

 ods of drouth to which the region is subject. 



The broad plains forming the summit of the plateau present for many 

 miles an almost featureless surface. The streams, so-called, are merely 

 wide, shallow stretches of gravel and sand, devoid of running water or even 

 pools except for short periods after the infrequent rains. Their courses are 

 generally comparatively straight, with very slight fall and practically with- 

 out banks. 



Ascending from the lower level over the steep grades or precipitous passes 

 of the escarpment the appropriate significance of the colloquialism, **on the 

 plains," is apparent. Far as the eye can see extend the unbroken, grassy 

 stretches of level surface, conveying an impression of boundless space com- 

 parable only to the great sandy deserts or to the oc^an. The only natural 

 features that vary the landscape are here and there broad, shallow depres- 

 sions or sinks, defined by the deeper green of their more luxurious herbage, 

 and perhaps the far-flung glistening ribbon of one of the waterless streams in 

 the distance. In the refracted light of the dazzling summer sun may be 

 seen the inverted images of cattle, buildings, fences and trees along the 

 horizon on either hand, mirrored in the phantom lakes of the mirage. 

 Scarcely more marked or impressive in the magnificent distances than the 

 few and slight inequalities of nature are the tangible evidences of civiliza- 

 tion: the ranch houses and stock sheds with the ever-present windmills, 

 many miles apart; fences and roads no less remote, and the grazing herds of 

 cattle that have come with the plainsmen and cowboys as successors to the 

 once numberless buffalo and the nomadic Indians, that not many years ago 

 held undisputed sway over these broad prairies. The traveler is some- 

 times startled into a realization of how very recent is the period of this 

 great cultural transition by coming upon the whitened skulls of the bison, 

 still occasionally to be seen about salt licks or along the streams, or by turn- 

 ing up the flint or obsidian arrow-heads of the primitive hunters. 



Underground solution of salt, magnesia and other mineral matter has 



resulted in the formation of the numerous shallow depressions that are in 

 many places so conspicuous a feature of the plains topography. These are 

 sometimes many acres in extent but seldom more than a metre or so below 

 the general level at their greatest depth, and as the sides slope gently to- 

 wards the centre they appear to be even shallower. Following the heavier 

 rains many of these sinks become shallow lakes, but this condition generally 

 continues for only a short time. Practically all of the water disappears in 

 a few days or weeks at most by evaporation, very little of it sinking to any 

 great depth into the ground. After the water is gone a rank growth of 

 coarse grasses and rushes springs up, and as the herbage on tlie level plain 

 becomes dry and brown the contrast in color is quite striking. 



The flora of the open plains is essentially grassy; species of Bouteloua, 

 Andropogon and Festuca being very abundant. Many annual and perennial 

 species of flowering plants are found but most of them are conspicuous only 

 for a short time after the rainy season. Low mat-like forms are common. 



