
+ —- 
GLACIAL PHENOMENA AND SUPERFICIAL DEPOSITS. 265 
and is somewhat lower than the general surface of the plain. Southward, 
it is said to extend about seventy miles, and finally to join the southern 
bend of the Souris River, where it gives issue to a small running stream. 
The present condition of affairs, however, wiil not serve to explain its 
formation, nor does it resemble the bed of an old tributary to the Souris, 
from its sudden northward ending, and the absence of systems of coulées 
ramifying fromit. It may be accounted for by supposing that it has 
been a former shorter channel of the Souris itself, but in that case, either 
its northern end must since in some way have been blocked up; or the 
river being dammed back in its own channel, has spread over the prairie 
in a lake-like expansion, which has finally found exit southward by the 
Riviere des Lacs Valley. The valley occupies just the position, which 
such flood water would take, and bends round the most eastern portion of 
the high ground, rising toward the third prairie steppe. 
613. Vallies such as those above described, appear, from the accounts 
of different explorers, to occur in many parts of the great prairie region,* 
and though some of them may tend to show the action of greater bodies of 
water than those now flowing in them, they cannot all be accounted for 
in this way. It is difficult to conceive any ordinary circumstance, which 
would cause a stream to leave a wide valley, often over 100 feet in depth, 
and to commence the formation ofa new channel of like proportions. 
The blocking of the stream by ice jams, or accumulations of timber, 
though capable of explaining change of course in rivers flowing through 
alluvial country, not much below the general level, cannot be supposed 
to be a sufficient cause for deflexion on so great a scale. A careful study 
of these phenomena, over great areas, may eventually bring to light some 
general cause, such as local, or unequal, elevation or depression. At 
present I can only account for these duplicate vallies, on the supposition 
that most of them are alternative channels, of the streams at present in 
existence. In a level region, composed of soft materials, it is probable 
that since the rivers first flowed across its surface, they have been subject 
from ice and timber, to frequent obstruction in certain places. Before 
the river bed had been deeply worn into the surface, the stream might 
thus easily be turned aside; and before many years, a similar occurrence 
in the new channel might cause it to revert to the old. By such oscilla- 
tion, a single river may cut out two or more beds simultaneously, and to 
nearly an equal depth. Some of these great dry vallies, may again 

* See especially U. 8. Geol. Surv. Territ., 1870. p. 176. 
