

GLACIAL PHENOMENA AND SUPERFICIAL DEPOSITS. 267 
nation some aqueous agency more potent than the present, and appear to 
resemble the disused beds of large tributary streams. To any one, how- 
ever, who has examined them in early spring, and during seasons of flood, 
their origin is apparent. The whole thickness of the soft prairie alluvium, 
is then completely saturated with moisture, and the coulées are brim- 
full with water, holding in suspension a great quantity of fine earthy 
matter, and flowing with a regular, though gentle and often scarcely 
perceptible current, toward the main stream. At their extreme rami- 
fications, little streams may be found, gathered together almost imper- 
ceptibly from the half-flooded surface of the prairie, and directed into a 
certain course—perhaps by means of snow-banks, which have not 
yet entirely disappeared—and just engaged in cutting through the 
tough prairie sod in the first process of the extension of the valley. 
617. During the post-glacial emergence of the country, every stream 
must at one time have flowed, as the Red River now does, into a great 
lake not much below its own level, and have been in the same way, more 
or less subject to floods and overflows. Many of the coulées now found 
bordering the river vallies in the higher prairies may date back to this 
time, and may not have since received important incremert. 
618. An examination of the beds of the rivers and streams, while 
probably leaving some balance in favor ofa period of greater rainfall, does 
not appear to offer any evidence of its great intensity. Other 
facts seem to point to the occurrence of a period when the rainfall was 
greater than at present, and it would even seem that a 
gradual dessication is yet proceeding over great areas of the 
west. This does not appear, however, to be more than can be 
accounted for by the decreasing area of forest—a subject elsewhere 
more fully mentioned. It is hardly probable that the prairies as a 
whole have been at any time wooded, but that large areas of forest have 
existed where bare plains now spread, is undoubted. The existence in 
some places of great quantities of the remains of land and fresh-water 
shells in the older river deposit of the Missouri, which now occur rarely 
if at all in the neighbouring region, has been mentioned by Dr. Hayden; 
and seeming to imply for the upper waters of that river at no very 
distant time, a considerable forest area, is one of the most striking 
facts in favour of a change of climate. Any very great or long 
continued excess of rainfall over the prairie region is, however, 
negatived by the appearance which its surface presents, and 
