Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 271 



valley trees already noted are present as in similar canyons without 

 the birch. 



Aside from the presence of the birch with its poorly developed 

 secondary layers the canyons are about the same as has been indi- 

 cated in the former association. The usual thickets of Prunus, 

 Symphoricarpos, Rhus, etc., are to be seen over the upper and outer 

 stretches of the canyons and these in turn soon give way to one 

 of the upland grass associations. 



The surface conditions in the birch sipringbranch canyons 

 are well described by Pound and Clements (57) as follows: "The 

 lower canyon sides and the banks of the streams are covered with 

 a very deep layer of humus and leaf mold, and are strewn with 

 fallen trunks of paper birch. Over all grows a uniformly dense 

 carpet of mosses, Funaria, Mnium, Bryum, and Hypnum, which 

 constitutes the sole layer of the deeper, wetter portions of these 

 canyons. Scarcely less characteristic than this dark green, mossy 

 layer is the well-developed layer of large fleshy fungi, Lactarius, 

 Clarkeinda, Helvella, Geoglossum, and Peziza, and of small cup 

 fungi, Sepultaria, Barlaea, Humaria, etc." 



The more open portions of these springbranch woodlands 

 where erosion has swept away the most of the humus, as on the 

 upper slopes and rims, are often characterized by a scattered, tufted 

 carpet of Selaginella densa. 



The Yellow Pine Formation 



This formation, with a distinctly western alliance, brings into 

 the state our only native pine, Pinus ponder osa var. scopulorum. 

 This is a variety of the western yellow pine which occurs in such 

 conspicuous abundance in the Rocky Mountains and the Black 

 Hills. The formation in our state is developed to the highest degree 

 upon the ridges and upper slopes of "Pine Ridge." Pine Ridge is 

 an escarpment which enters the state near the northwest corner 

 from Wyoming and extends eastward as a narrow tongue approxi- 

 mately parallel with the northern state line. This northerly facing 

 scarp cut through the Loup Fork Beds has been greatly eroded so 

 that the "ridge" is in reality a complex of deeply cut canyons, often 

 with precipitous sides, varying from a mile to several miles in 

 length, which originate in the high short-grass or spear-grass covered 

 plains to the southward and make their way through the scarp at 



