CHARACTERISTICS AND DISTRIBUTION OF LAKES 633 
Madison stands between this and the smaller Lake Monona, both of 
which are traversed by the Catfish River, an affluent of the Rock River, 
which joins the Mississippi a little below Davenport. Lake Mendota 
is interesting chiefly from the fact that a biological station has been 
established on its shores, and that observations have been made on 
the conditions of life and the distribution of carbonic acid in its 
waters.^ Excellent bathymetrical maps were published of this and 
other lakes of Wisconsin by the Wisconsin Geological and Natural 
History Survey in the years 1899-1901. 
The Mississippi has its rise in the small Lake Itasca (lat. 47° N., 
long. 95'' W.), in a morainic region. Its upper course is guided mainly 
by the irregular deposits of drift over the immature land-surface, and 
it receives the waters of the countless lakes lying in the depressions in 
Minnesota and Wisconsin. An indication of the renewed youth of 
the river is seen in the number of falls and rapids in its course, and in 
the mode of formation of Lake Pepin, a short distance below St Paul. 
Lake Pepin occupies a barrier basin, the result of a lateral stream 
carrying more detritus into the valley than the main stream can get 
rid of. As described by G. K. Warren,- the excess of material 
brought down by the Chippeway River to the Mississippi obstructs 
the main stream so as to cause an expansion of its waters. The lake 
is shallow from overflowing on to an open valley, and is about 28 miles 
long by nearly 3 miles wide. 
An approximation to the same conditions occurs at the junctions 
of the Wisconsin and Illinois Rivers with the Mississippi, but in 
these instances it is only in the low-water stage that the ponding 
becomes conspicuous. 
South-Eastern Missouri affords several examples of lakes formed 
by local subsidence resulting from earthquakes. In 1811 and 1812 
a large area of the Mississippi valley was shaken, and several parts 
were depressed so as to be submerged to a small depth by river 
water. Lake St Mary is the largest of these submerged tracts, and 
measures^ 30 miles in length, by 5 to 7 miles in breadth. 
The limestone regions of Kentucky have been hollowed out 
through successive ages into many caverns by the chemical action of 
rain-water on the joints of the rocks and by the erosive power of 
streams. The Mammoth Cave in Kentucky (lat. 37° 14' N., long. Mammoth 
36° 12' W.) is from 40 to 300 feet high, and has vast chambers Kentucky. 
^ See E. A. Birge, "Plankton Studies on Lake Mendota," Trans. Wisconsin 
Acad. Nat. Sci. (Madison), vol. x. p. 421, vol. xi. p. 274, 1897-98 ; " The Respiration 
of an Inland Lake," Pop. Sci. Montlikj (New York), vol. Ixxii. p. 337, 1908. 
- Amer. Journ. Sci., Ser. 3, vol. xvi. p. 420, 1878. 
2 Humphreys and Abbott, "Report on the Physics and Hydranlics of the 
Mississippi," Professional Papers, Corps of Engineers, U.S.A., 1861, Plate 11. 
