MUSSELS OF CENTRAL AND NORTHERN MINNESOTA. 7 



that of the Red River of the North, a large lake, called Lake Agassiz, 

 was formed along the edge of the ice. As the ice withdrew, this lake 

 extended northward along the Red River Valley as far as what is 

 now Lake Winnipeg. Lake Agassiz was 700 miles long and covered 

 110,000 square miles, and its duration is estimated to have been 

 about a thousand years. Its outlet, known as the glacial river 

 Warren, was along the valley now occupied by Lakes Traverse and 

 Big Stone and the Minnesota River, into the Mississippi. At that 

 period, therefore, the Mississippi River may be said to have had its 

 origin in Lake Agassiz. And during the thousand years of its 

 existence, this lake had abundant opportunity to become populated 

 with fresh-water mussels ascending from the Mississippi. The lake 

 disappeared, however, and the valley of the Minnesota River sepa- 

 rated from that of the Red River. But the mussel faunas of the two 

 rivers were derived from identically the same source, and the only 

 differences would be the few minor changes that have crept in since 

 then. 



Many other glacial lakes were formed during the melting of the 

 ice sheet, only to be subsequently obliterated. But while they 

 existed they served as temporary channels for the migration of fish 

 and mussels from the south. Two of these are of especial interest, in 

 view of the results of our present investigation. Lake Upham once 

 covered the region south of the Mesabi Iron Range, and drained 

 southeast along the valley now occupied by the St. Louis and Cloquet 

 Rivers. This lake was very shallow and temporary and, as far as 

 can be determined, was not connected with the Mississippi River in 

 any way. This fact accounts for the paucity of mussels in that 

 portion of the State, as well as in all the Lake Superior drainage. 



Lake Nicollet was formed around the headwaters of the Mississippi 

 River and covered an area of 1,500 square miles. It drained south- 

 west into the valley now occupied by the Crow Wing River and 

 thence into the Mississippi. The Crow Wing River thus becomes the 

 modern representative of the old headwaters of the Mississippi, and 

 is older than that portion of the latter river which now extends from 

 the mouth of the Crow Wing up to Lake Itasca. These facts explain 

 the richness of the mussel fauna in the Crow Wing and its tributaries. 

 At two places in the area covered by the present survey the bed rocks 

 appear above the drift with sufficient prominence to materially affect 

 the mussel distribution. At Carlton the St. Louis River plunges 

 over a rocky ledge and through a gorge, which prevents the passage 

 of either fish or mussels. Consequently that portion of the Lake 

 Superior drainage which is tributary to the St. Louis River above 

 Carlton is devoid of mussels. 



