604 THE GLACIAL LAKE AGASSIZ. 



No tree of exclusively western range extends east to the Reci River 

 basin, and it has only a few western species of shrubs, of which the most 

 noteworthy are the alder-leaved Juneberry or service berry (in Manitoba 

 commonly called "saskatoon"), the silverberry, and the buffalo-berry. To 

 these are also to be added the shrubby Oenothera albicaulis Nutt., which 

 occurs chiefly as an immigrant weed, and the small-leaved false indigo, 

 which abounds on moist portions of the prairie. The silverberry (usually 

 called "wolf willow" in the Red River Valley) is common or abundant from 

 Clifford, N. Dak., and from Ada, Minn., northward, forming patches 10 to 

 20 rods long on the prairie, growing only about 2 feet high and fruiting 

 plentifully, but in tliickets becoming r» to 10 feet high. Its silvery whitish 

 foliage and fruit make this shrub a very conspicuous and characteristic 

 element of tlie Red River flora. 



The single species of true sagebrush belonging to this basin (Artemisia 

 cmia Pursh) extends east in North Dakota to the Heart Mound, (J miles 

 northwest of Walhalla, or 35 miles west of the Red River at Pembina, and 

 to a hill close west of the Sheyenne River, about 8 miles south of Valley 

 Citv, growing in botli places on outcrops of the Fort Pierre shale. It 

 attains a height of 1 to 3 feet, and the tough wood of its base is 1 to li 

 inches in diameter. Artemisia frigida Willd., called "pasture sagebrush" 

 by Macoiui, is abundant throughout a wide area westward, extending east 

 locally to "the ridge" east of Emerson, Manitoba, the Falls of St. Anthony, 

 and Lake Pepin. 



Causes of run if at ion of the forest. — The boundary between the forest and 

 the prairie, shown by PI. XXXVIII, and the similarity of the two regions 

 in their topographic features and drift deposits, have been noted in Chapter 

 II (pp. 44-46). The usually aln-ujjt transition from the tim1)ered to the 

 prairie country and the general absence of trees and sln-uljs in tlie prairie 

 region have been often attributed to the effect of fires. Through many 

 centimes fires have almost annually swept over these areas, generally 

 destroying all seedling trees and shrubs, and sometimes extending the border 

 of the prairie by adding tracts from which the forest had been Inirned. 

 Late in autumn and again in the spring the dead grass of the prairie burns 

 verj- rapidh', so that a fire within a few days sometimes spreads 50 or 100 



