49 Canadian Record of Science. 
during these depressions the subsidence was sufficient for, 
or the other surrounding conditions favourable to, the 
action or even the existence of icebergs, though previous to 
this time, this section of the Northwest was no doubt also 
subject to the action of ice, all evidence being now covered 
up by the more recent deposits here referred to. 
West of these lower and more recently formed prairies, 
are the rolling prairies, which have an origin somewhat 
different. The stretches of sand, both on the surface and 
under the clays, point to the existence of extended lake and 
sea margins at more than one period. The extensive, some- 
what parallel gravel ridges at Arden, the gravel knolls, the 
smaller ridges with boulders in and on them at Birtle and 
west of Langenburg, and the uneven, rolling nature of the 
surface of the prairie, all seem to me to point to the action 
of icebergs in the glacial or post-glacial seas, modified after- 
wards by the water during subsidence, and to indicate the 
direction of the force, whether wind or current or glacier, 
which at these places impelled the bergs onward. Further, 
the thinner surface loam, mixed to the westward with some 
sand, would seem to point to a condition of growth and 
decay of plant life, less defined than and probably of a 
different character from that on the lower prairies to the 
eastward. 
The Assiniboine, though presently a branch of the Red 
River, was not always so, and is in its upper reaches above 
Brandon, a much older river. When the whole prairie east 
of the Riding and Pembina Mountains was a vast shallow 
lake, the Assiniboine was a large stream varying from half 
a mile to a mile and more in width for most of its course, 
discharging into this lake the surplus waters of the country 
to the northward and westward. As the whole surface 
of the continent here, to the east and west, but more 
especially to the westward, continued to rise, in the long 
lapse of time, the Assiniboine, with the strongly increased 
current which its relatively higher level westward gave it, 
cut its way through the surface soils to its present great 
depth of about two hundred feet below the prairie level. 
