53 
of the post-Glacial lake that stood at about the 800- foot level. Conse- 
quently it may be correlated in age and origin with the Salt Plain prairies. 
At the withdrawal of this lake the clay deposits on its bottom must have 
acquired plant cover of low stature that was either similar to the present 
one or has developed into it. It has persisted longest on the river side of 
the plain, being colonized from the northwest by willows and aspens, and 
shut away from the river below the present site of the ranger station by 
the development of a flood-plain timber on river-deposited soils. 
A few sink-holes in the plain have fluctuating but constant water 
supplies. Local inhabitants state that the water levels change with the 
seasonal changes in the river, but this was not verified. Typical wet meadow 
and shore marsh vegetations are developed, involving Carex rostrata, Beck- 
mannia Syzigachne, and Glyceria pulchella, and a common association on 
the drier margins consists of Hordeum jubatum and Potentilla Anserina. A 
shrub zone with Salix Bebbiana and Symphoricarpos occidentalis as primary 
species separates the sloughs from the surrounding prairies or aspen woods. 
THE SALT PLAIN PRAIRIES 
The greatest extent of open prairies in the park is on the Salt Plain, the 
name used in this paper to designate the level country lying just west of 
Slave river. Its boundaries have already been defined or approximated, 
and its general topography described in another section, so they will not be 
discussed at length here. The plain is regarded as part of the bottom of 
what has been called the 800-foot post-Glacial lake (8), its clay soils being 
derived from deposits in this lake. As indicated elsewhere, it is broken by a 
few morainic ridges, the exact positions of which have not all been mapped. 
The most prominent of these lies in a broad arc extending eastward from the 
vicinity of Heart (Raup) lake. The aerial photographs show clearly the 
positions of the ridge, which is composed largely of sand, and also the topo- 
graphy of the neighbouring salt plains to the northeast of it. These have 
broad, shallow depressions separated by minor ridges that are more or less 
parallel to the main one. 
It is thought by the writer that this ridge, as well as the lesser ones 
northeast of it, is a member of the lobate morainic system of which the 
Ninishith hills and the tangle of ridges to the south and southeast of them 
form the terminus. With the recession of the glacier from the terminal 
moraine, the waters of the 1,100-foot lake were lowered as far as the passes 
in the moraine would permit. Then a series of lower lakes was formed as 
the ice front receded to the northeastward. One of these seems to have 
occupied part of the country southwest of the Salt Mountain escarpment, 
and to have accumulated as bottom deposits such clayey materials as occur 
in the Grassy Slough district. Another, or an eastward extension of the 
same one, was probably formed in the part of the Salt Plain which lies 
southwest of the ridge above described. The recession of the ice from the 
latter moraine and its near members may have left all these higher lake 
bottoms exposed or it may have maintained the lakes and simply formed 
new settling basins still farther to the northeast and north in which the most 
recently formed soils may be found. From these notes it seems that 
instead of a single lake level at about 800 feet, there may have been two 
or three lakes whose levels ranged in the vicinity of 800 and 900 feet. These 
