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 flow. 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 and the extreme 

 upper portion now occupied by lake Traverse served as a 

 head lake for the northern 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 



