580 METASPERMAE OF THE MINNESOTA VALLEY. 
much of the glacial till of both epochs was washed away thus 
exposing the older crystalline rocks of the upper region as 
they are now seen protruding from the floor of the valley. In 
this region of the crystalline rocks it is easy to imagine how 
turbulent must have been the river Warren, as Upham has 
named it, in its tow. When the ice finally retreated beyond 
Hudson Bay the drainage of the lake Agassiz region set 
towards the north, as it remains to the present. A divide 
appeared in the old gorge of the river Warren andthe extreme 
upper portion now occupied by lake Traverse served as a 
head lake for the nerthern trending waters, while the great 
extent, from the head of Big Stone lake to the mouth of the 
present river, was used by a much diminished stream, the 
Minnesota river of modern times. 
During both the first and second post-glacial periods, when 
the Minnesota gorge was draining to the sea large bodies of 
fresh water which had resulted from the melting ice, it was’ 
eroded to a much greater depth than to-day. The gorge of 
modern times is about one-half filled with the more or less 
modified till of the two epochs and the alluvial deposits of the 
interglacial and final post-glacial periods. At Belle Plaine, for 
example, as reported by A. Winchell, in a well dug on the 
bottom-lands of the gorge the rock was found 170 feet below 
the present surface of the river. This indicates, then, an 
erosive action having made itself felt at almost four hundred 
feet below the present general country surface. The river 
Warren, after its waters had ceased to carry and deposit modi- 
fied drift became, as Upham has shown. ‘‘a powerful eroding 
agent,” and doubtless at this period the gorge was cut to its 
greatest depth. Since the diminution of the stream owing to 
the disappearance of lake Agassiz, the tributaries have 
brought in considerable silt and by the deposition of this silt 
by the different streams the gorge has come again to be partly 
filled with alluvium. The Lac Qui Parle river has thrown a 
dam of sediment across the channel of the present Minnesota 
and this has formed the back-water lake‘ known as Lac Qui 
Parle. The sluggishness of the Minnesota at its mouth, and 
for thirty miles up stream, is in a like manner due to the sedi- 
ment thrown across its mouth by the Mississippi. 
In Blue Earth county a smaller glacial lake existed which 
drained into the Des Moines river by Union slough, and perhaps 
also, at other times, into the river Warren by way of the Blue 
Earth river gorge. Whether the river Warren at any time 
