3H 



SCIENCE 



[Vol. XXI. No. 540 



SCIENCE: 



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THE FORMATION AND DEFORMATION OF MINNESOTA 

 LAKES. 



BY C. W. HALL, MINNEAPOLIS, MINNESOTA. 



In respect to inland waters Minnesota shows conditions which 

 are exceptional in North America. Exclusive of that portion of 

 Lake Superior within her houndary, the State has 5,700 square 

 miles of lakes varying from a few acres to the size of Red Lake, 

 which has 340,000 acres in its area. These lakes are conveniently 

 divided into three classes. 



First, rock-bound lakes. These bodies of water occur chiefly in 

 the northeastern portion of the State. They occupy the troughs 

 in the crustal folds that have contorted the surface, or the de- 

 pressions where excessive faults have broken and considerably 

 tilted the strata. These, as a rule, are long, narrow, and deep. 

 The water is clear and sparkling, abounding in fish, and remark- 

 ably free from the various forms of plant growth peculiar to 

 shallow waters. 



Second, silted-river lakes. These lakes occur in rivers where 

 rapid streams have brought down a deposit into the channels of 

 the n3ore sluggish ones, the silting debris being so course that the 

 slower current fails to transport it. Thus slower streams are 

 choked up and the water set back for miles. Such lakes are Lake 

 Pepin, formed where the sluggish Mississippi is dammed by the 

 debris brought into its channel by the more rapid Chippewa River, 

 Lake Saint Croix, Lake Lac qui Parle, and many otliers. 



Third, glacial lakes. If this group could be minutely subdi- 

 vided, there would be seen several types of lake formation. Those 

 of the Lake Agassiz type, where one portion of the shore con- 

 sisted of a wall of ice, have long since disappeared and left scat- 

 tered pools of varying sizes, occupying the depressions in the 

 generally level surface of the old lake bottom. Going outside of 

 the State for an illustration, we may name Lake Winnipeg as the 

 largest pool now remaining in the bottom of glacial Lake 

 Agassiz. Many other lakes are scattered over the level portions 

 of the State, occupying the depressions in the drift-sheet where 

 this was laid down evenly through the steady and uniform move- 

 ment of the ice, or through the silting effect of waters due to the 

 melting of the ice border. 



But by far the greatest number of lakes in Minnesota are those 

 occupying the depressions in the unevenly distributed morainic 

 matter deposited during glacial times. Portions of the State are 

 thickly studded with these beautiful sheets of clear water. The 

 region between Minneapolis and the Red River valley is appropri- 

 ately called the Lake Park Region, as lakes occur here in vast num- 

 bers. Wright County, nearest to Minneapolis, contains 259 lakes, 

 Kandiyohi County contains 286, and Ottertail County, well up 

 towards the borders of glacial Lake Agassiz, holds the banner 

 over 430 lakes. Passing northeastward from the Lake Park 

 Region towards Ontario, past the head-waters of the Mississippi, 

 and across the upper streams of the St. Lawrence River basin, we 



pass gradually from the region of moraines to the region of thin 

 glacial deposits and constant and tumultuous rock-exposures, 

 carrying large numbers of lakes of the first type. 



It has been frequently estimated that Minnesota contains 10,000 

 lakes. To one passing over the State through the region named 

 this does not seem an exageration. There must be several thou- 

 sand lakes from one mile in length upwards to the very largest. 



While glacial lakes show many varieties of form due to the 

 position of tongues, branches, and subdivisions of the moraines, 

 they are generally circular in outline. The deepest portion is at 

 the centre. Their shores show but few successive beaches to in- 

 dicate erosion at their outlets and consequent drainage, or great 

 variation in the amount of rainfall during the last few thousand 

 years. They were all evidently formed in the same general 

 manner, by the washing down of fine silt from the high land into- 

 the bottom, thus gradually filling the interstices in the gravels and 

 sands beneath them, making water-tight bottoms to hold the 

 water. Probably the lakes were small at first, and enlarged 

 gradually as this deposition of fine silt extended their borders 

 until the lowest point in the margin was reached and an outlet 

 drained away the excess of water. When this outlet was reached 

 the conditions of formation ceased, and the conditions of deforma- 

 tion became manifest. Material was constantly washing in from 

 the high lands around through the melting snow of successive 

 springs and the heavy showers and rain-storms of the summer 

 months; it was also brought by streams flowing into the lakes 

 from every direction, and formed m situ by the vigorous growth 

 of aquatic vegetation. In the shallower lakes this last cause of 

 deformation works with great rapidity. After the ice disap- 

 pears in the spring under the warm sun of this latitude the water 

 very rapidly rises in temperature to 70" or 75°, a favorable tem- 

 perature for vigorous vegetable growth, and thus plant-forms 

 which can get foothold upon the lake bottom will develop a vast 

 amount of plant debris. Already hundreds of the shallow, small 

 lakes of the State have disappeared, and rich, productive hay- 

 meadows have taken their place. This will be the fate of thou- 

 sands more within the coming century. On every hand we hear 

 old settlers speak of large lakes once affording superb hunting 

 ground for wild geese, ducks, and other water fowl and excellent 

 fishing, as now either hay-meadows or extensive marshes soon to 

 be fitted for hay production by a fui-ther lifting of the surface 

 above the level of the outlet through this rapid accumulation of 

 the vegetable mold. 



The lakes of Minnesota afford some of the most attractive sum- 

 mer resorts to be found in our northern States ; as already famous 

 can be mentioned Minnetonka, White Bear, the Chisago Lakes, 

 and Waconia. The list can be indefinitely extended. Their 

 climatic influence is very marked. The amount of heat stored 

 up during the summer, if calculated by its mechanical equivalent, 

 is enormous. With 48° as the average increment in temperature, 

 10 feet as an average depth of 5,700 square miles of water sur- 

 face, we have nearly 11 cubic miles of water. Since each cubic 

 foot of this water receives 1,250,000 foot pounds of heat, which 

 must be given off during the autumn months as these lakes grad- 

 ually settle down to the freezing-point of winter, the amount of 

 heat thus made available for our autumn weather reaches quin- 

 tillions of foot pounds. This warmth is a break against early 

 autumn frosts. The south side of Lake Minnetonka has most; 

 productive vineyards and fruit gardens, while the northern side 

 is liable to early frosts. 



Other writers have called attention to the distribution of fresh- 

 water lakes. They are almost wholly confined to the glacial 

 regions of our globe. Northern Europe and northern central 

 North America, with other isolated portions of the globe, are the 

 only places where many bodies of fresh water are found. It re- 

 mains to be noted that within these glaciated regions the oldest 

 portions are already comparatively free from lakes. The southern 

 border of the glacial area of the United States is almost wholly 

 devoid of them. The vast prairies of Ohio, Indiana, and Illinois 

 have but few, yet tliere are vast agricultural tracts within these 

 States which show deep and rich accumulations of vegetable 

 mould, evidence of former aquatic plants. Doubtless there were 

 once thousands of lakes within these States, but the silting-in of 



