272 MINNESOTA BOTANICAL STUDIES 



approximately right angles to the general trend of the scarp. The 

 heads of these canyons are sometimes several hundred yards above 

 the outlets from which flow numerous clear streams northward 

 toward Hat Creek and White River. 



The maximum development of the yellow pine formation is to 

 be found upon such canyon sides as they occur in Sioux and Dawes 

 counties. Similar stands occur farther south in Scottsbluff and 

 Banner counties but these areas do not enter the sandhill region. 

 Eastward from Sioux and Dawes counties the abundance of the 

 yellow pine rapidly decreases as the "ridge" becomes lower and 

 narrower. The species is, however, found in great frequence, and 

 is often locally abundant much farther eastward upon the extremely 

 xerophilous ridges and cliffs of the Niobrara River and its tribu- 

 taries. The species continues in gradually diminishing rate across 

 Cherry county and as far eastward as Brown and Keya Paha 

 counties, about 280 miles east of the Wyoming line where it enters 

 the state. A remarkable correlation exists between the soil of the 

 Loup Fork Beds and the distribution of this tree. The greatest 

 expression of the species is always seen where there is the greatest 

 quantity of these rocks exposed or where they lie near the surface. 



Toward the eastern limits of its distribution along the Niobrara, 

 the western yellow pine meets and mingles with the species of trees 

 from the central hardwoods region of the east that have already 

 been noted as occurring in the canyons of the Niobrara and its 

 tributaries. Thus we have the meeting and overlapping of slender 

 arms of western and eastern forest centers along the northern border 

 of the sandhills. 



The yellow pine enters the sandhills by way of a number of 

 the springbranch canyons which reveal also the characteristic 

 broadleaf features described on preceding pages. One of the best 

 instances of the entrance of the pine is seen along the Snake River, 

 a stream which arises in eastern Sheridan County and flows eastward 

 across northern Cherry County to a point somewhat east of the 

 center of this county, where it turns sharply northward and cuts 

 its way through the underlying rocks to the Niobrara. Along its 

 lower course the Snake has cut a deep, narrow canyon with walls 

 of Loup Fork rocks almost vertical in many places. Such conditions 

 are found especially below the falls of the Snake about 30 miles 

 southwest of Valentine. At this place the clear rushing stream about 

 50 feet in width leaps over a ledge in its bed and produces a waterfall 



