THE NEBRASKA SAND HILLS. 283 



eragrostis (Eragrostis trichodes). From the appearance of these grasses 

 the decline of the blow-out is rather rapid. As these various species wan- 

 der up the steep sides, and the force of the wind striking upon the upper 

 slopes is reduced and the sand held from blowing, other species wander 

 in from the bunch-grass association. If these new plants are properly 

 provided with rhizome device like that of their predecessors they soon 

 begin to weave themselves into the now conspicuous blow-out association. 

 The plants that most commonly gain entrance soon after the grasses 

 have become well established are prairie pink (Lygodesmia juncea), 

 small-flowered psoralea (Psoralea micrantha), painted pod (Phaca longi- 

 folia), and the hairy golden aster (Chrysopsis villosa). Indeed, some of 

 these species may get a start in the declining blow-out almost as soon 

 as Redfieldia. 



In this manner the effect of blow-out conditions are finally so far 

 removed that the bunch-grasses enter and take possession of the area 

 so well prepared by the pioneers in the succession. It is almost pa- 

 thetic to find that Redfieldia, the first plant to appear in the blow-out 

 and the plant of gi'eatest importance in its reclamation, after struggling 

 against severe physical conditions for so many years is also the first to 

 disappear. It fails in the competition with the later arrivals and is 

 then forced to find other blow-outs in which it may continue its great 

 work. With the development of the bunch-grasses and the resulting 

 competition all of the blow-out grasses disappear, and then with the in- 

 coming of the characteristic members of the bunch-grass association, 

 the change from blow-out to hillside is complete. The only indications 

 of the former history of the place are seen in the grassed-over crater 

 which frequently persists as a characteristic form, and perhaps a few 

 straggling clumps of the blow-out grasses lingering in the near vicinity. 



On the lower slopes of the hills and in the valleys many new species 

 are encountered as the bunch-grasses of the uplands are left behind. 

 Rather low down on the north-facing slopes one frequently finds con- 

 spicuous associations of willows (Salix humilis) and dogwoods (Cornus 

 stolonifera). The prairie shoestring (Amorpha canescens) also gives 

 tone to the lower slopes in many places by its typical low-branching, 

 ashen-colored plants closely aggregated. Even the taller shoestring 

 (Amorpha fruticosa) occasionally wanders from its usual habitat in the 

 moist valley and is found on north slopes among the willows and dog- 

 woods. The presence of such plants always indicates a higher percent- 

 age of soil moisture quite near the surface than is found typically in 

 the bunch-grass association. The explanation of this phenomenon is not 

 hard to find, because such associations and such soil conditions almost 

 always mark an outcrop of clay or other impermeable rock strata "which 

 lead the ground water from under the hills in a horizontal direction until 

 it is brought near the surface. If the clay or rock does not actually ap- 

 pear on the surface it is usually found a few feet beneath, so that the 

 effect is practically as has been given. The water is frequently bo 

 abundant in such situations that it seeps out and collects in cow tracks 



