378 Wisconsin Academy of Sciences, Arts, and Letters. 
pebbles. The remainder, which constitutes about 60 per cent, 
of the total, is made up of gravel with an admixture of boul¬ 
ders of various sizes. The sand beaches are situated chiefly 
in the bays while the rock beaches are found at the most ex¬ 
posed points. 
Bottom .—The bottom of the lake in the littoral area usually 
consists of coarse material, such as sand, gravel, and boulders 
of various sizes; rarely it is made up of mud which is covered 
with a thin layer of sand as in University and Catfish bays. 
Some detached shoals possessing rugged bottoms are of some 
ecological interest also. A shell zone several meters wide 
marks the lower limit of wave action. This zone is found at a 
depth varying from 4 meters in less exposed areas, such as 
University Bay, to 7 or 8 meters on the steeper shores; at Pic 
nie Point, probably the most exposed point on the lake, it lies 
at a depth of 14 meters. Usually the substratum preceding 
the shell zone contains a large percentage of marl, but in the 
zone proper shells constitute almost 90 per cent of the bottom 
deposit. 
In general the bottom is nearly pure mud below 8 meters, 
but in the more exposed places a considerable admixture of 
sand may extend to a depth of 10 or more meters. 
Sources of water .—Lake Mendota receives its water supply 
through several creeks and the Yahara River and from a num¬ 
ber of springs. Some of the springs are of fair size and of 
considerable local interest; they are situated chiefly at the 
west end of the lake. 
B. ECOLOGICAL HABITATS. 
The lake as a whole may be divided into three general re¬ 
gions,—^the littoral, or phytal region; the aphytal region; and 
pelagic region. The bottom of the lake may be best classed as 
follows: The littoral, with its two subdivisions, the eulittoral 
(euphytal), and the sublittoral (dysphytal) ; and the aphytal 
area. (See table 1). 
a. The Littoral Region. 
In Lake Mendota the ecological, physiological, and physio- 
graphical features are to a great extent parallel. The plant 
zone, the littoral area, and the zone of photosynthesis are ap- 
