6 MUSSELS OF CIENTEAL AND NOETHERN MINNESOTA. 



road, stopping at convenient centers and driving to the lake or river. 

 During this trip 45 lakes and 15 rivers were examined, with the re- 

 sults as herein given. 



Samples of the shells obtained by the authors at the different lo- 

 caHties herein mentioned have been identified by Mr. H. Walton 

 Clark, of the biological station at Fairport, Iowa. Similar samples 

 have been tested as to their mercantile value by Mr. John Southall, 

 of the same station. 



In order to understand the geographic distribution of the mussels, 

 it is necessary to review so much of the glacial geology as will explain 

 the present location of the lakes and rivers as well as their former 

 relations. 



GLACIAL ORIGIN OF LAKES AND RIVERS. 



Minnesota was formerly covered by a great glacier or ice sheet, 

 which came down from the northeast and flowed across the State 

 into Iowa. When the glacier melted, the gravel, sand, and clay 

 which it carried was spread out over the surface of the underlying 

 rocks and has remained there ever since. The only exception is a 

 narrow strip bordering the Mississippi River from Lake Pepin south- 

 ward, and thus outside of the present survey. North of Lake Superior 

 this drift, as it is called, is thin in places, but elsewhere it averages 

 from 100 to 150 feet in thickness and effectually conceals the bedrock. 



Both glacier and drift have made the contour of the State more 

 level and uniform than it was before. The glacier accomplished this 

 by grinding off the ridges and elevations, while the drift assisted by 

 filling in such hollows as were left by the glacier. In some places 

 the material of the drift has been subsequently worked over and 

 arranged in layers by the streams and rivers that flowed from beneath 

 the glacier, but in most places it is still unstratified. During the 

 melting of this great glacier there were successive periods of advanc- 

 ing, halting, and retreating, and at such times the drift accumulated 

 in long hills or ridges called moraines, parallel with the edge of the 

 glacier. Twelve of these moraines may be traced across the State 

 in various directions, and while they are only from 25 to 75 feet in 

 height, they are still sufficient to determine the general drainage 

 of the State. They have the further effect of rendering any free 

 drainage impossible, and thus they give rise to the remarkable number 

 of lakes that are found in Douglas, Ottertail, Itasca, and other coun- 

 ties. Furthermore, the streams and rivers are very winding, and have 

 a uniform current, and there are plenty of sand and gravel beds in 

 the bottom of the lakes and rivers, and not very much mud. All 

 these conditions are favorable for mussel growth and propagation 

 and profoundly influence the mussel fauna. 



When the melting ice sheet had receded beyond the moraine or 

 watershed which separated the basin of the Minnesota River from 



I 



