52 
In its zones of contact with neighbouring woods the prairie is slowly 
being invaded by a group of shrubs that make a dense tangle over large 
patches. 
Primary spp.: Rosa acicularis 
Symphoricarpos occidentals 
Agropyron Irachycaulum var. unilaterale 
Salix Bebbiana. (in clumps) 
The secondary species are drawn largely from the more mesophytic types 
in the prairie flora, wdth a few such as sapling aspens from the woods farther 
back from the river. 
The gradual transition to the woods is finally consummated by the 
dominance of the aspens and the presence of a few accompanying species 
such as the red raspberry Rubus idaeus var. canadensis and the green orchid 
Habenaria viridis var. bracteata. This rather open aspen woods is the 
most advanced timber on the lower plain. On the next higher level to the 
northward is a much heavier growth of larger aspens, with much greater 
humus accumulation, the dominance of the grey willow in the shrub layer, 
and a general elimination of the prairie species. 
The gypsum bluff at the river is of varying slope, in some places pre- 
cipitous, and in others more gentle with a mantle of soil that has fallen down 
from above. The commonest and most characteristic species are Artemisia 
fngida, Elaeagnus argentea, and Hedysarum alpinum var. a mericanum. 
Among the secondary species, the cinquefoil Potentilla Anserina , lettuce 
Lactuca puchella, and Indian hemp Apocynum sibiricum are not found on 
the prairie. About twenty species are derived from the latter, wiiereas the 
sand-bar willow Salix interior var. pedicellata represents the river margin. 
On the steepest places Artemisia fngida is almost the only plant. The older 
bluff at the north side of the plain has much more soil on it, although in 
some places this is a sliding, whitish material, weathered out of the under- 
lying gypsum. Where better soils have fallen from the top and covered 
the slope, a thicket of aspens and service berries Amelanchier fl-orida has 
attained a foothold. It has a scattered mixture and shrubs and herbs from 
both prairie types. Less stable soils have open associations in which 
Artemisia frigida , Arctoslaphylos Uva-ursi, Astragalus adsurgens , and 
Amelanchier florida are the most prominent species. They have with them 
various herbs from the driest parts of the prairie. 
The sand-bar deposits on the lower side of the point have been forming 
there since the river commenced cutting its present channel. The topo- 
graphy and vegetation of these bars, new- and old, are shown on Figure 
11, B, C, and will be discussed in detail in connexion with local river 
flood-plain deposits. The sharp contrast between the timber on the oldest 
bars and the herbaceous vegetation of the adjacent prairie is striking 
(Figure 5), and raises the question of the possible origin of the prairie. 
Explanations involving fires, clearing, or varying local conditions of 
exposure seem quite inadequate, since burning or clearing would hardly 
be so well confined to prairie spaces between the heavy spruce timber of 
the river deposits and the encroaching aspen clumps. If exposure to winds 
of the broad valley of the river were significant, then the prairie could 
not be expected to persist so far inland w r here it is protected by the timber. 
There remain only soil and moisture factors. The prairie stands at about 
750 feet above sea-level, and is probably growing on a portion of the bottom 
