64 AQUATIC CONDITIONS 
TABLE V (76) 
Pressure in 
Atmospheres 
Depth in meters.| 10.328 20.6 
It will be noted that there is a little more than one atmosphere 
increase in pressure for each 10 meters (33 feet) in depth because water 
is very slightly compressible. According to this, animals in the deepest 
parts of Lake Michigan are living under a pressure of about 375 pounds 
to the square inch. 
5. BOTTOM 
The character of materials and topography of the bottom are very 
important to animals living on the bottom, but it has its effect also on 
free swimming animals as a determining factor in the amount of sedi- 
ment. 
The kind of bottom is important because many animals are 
dependent upon solid objects for attachment and are absent from 
bottoms made up of fine materials. Others must burrow into mud 
or creep on sand and gravel. This will be discussed later in special 
cases, particularly in streams. 
Topography of the bottom in shallow water is important in lakes 
locally in affecting wave-action and currents, and through these, bottom 
vegetation and temperature. Ward (75) noted such effects but did 
not carry the work far enough to solve any of the problems involved, 
which are usually local. In lakes, bottom materials are most important 
in shallow water, because of their effect in connection with wave-action, 
the amount of sediment in suspension, and the stability of the bottom. 
The bottom materials of lakes vary greatly locally. Taking Lake 
Michigan as an example, if we were to see the region about Chicago 
denuded of all vegetation, we would be able to appreciate the fact that 
there are bowlder deposits, gravel deposits, sand, clay, and bare rock. 
Evidently the ice sheet left the same kind of bottom materials strewn 
with the same irregularity in the bottom of the lake as on the land. 
Apparently wave-action has not affected them below 25 meters (85 feet). 
The waves of Lake Michigan are believed not to move sand below 
9 meters (30 feet). It is thought that, during the Champlain stage, the 
lake stood at a level 60 feet below its present level. Along the north 
shore there is a cliff at this level with sand deposits lying on the side 
toward the deeper water. Inside of this is an area of clay and then, next 
