30 NN. M. FENNEMAN 
east coast of Florida, is perhaps ten fathoms in the first two 
miles, but if the sea level should fall ten fathoms, or the land 
should rise by that amount, the new ten-fathom line would lie 
many miles off-shore, and new barriers might be expected. On 
some of the small lakes of Wisconsin, especially those without 
outlet, as Silver Lake of the Oconomowoc group, the falling 
level has found a deficient slope and barriers are constructed. 
The front of the marginal shelf—lIf the marginal shelf be a 
pure wave-cut terrace with no addition by deposit, its limit will 
be marked by an angle where the plane of the shelf meets the 
original bottom. The depth of the shelf at this edge will con- 
stantly approach wave-base, for it may be safely assumed that 
wherever waves can agitate, there will be sufficient current to 
transport. If there are currents strong enough to erode below 
wave-base, the shelf may be cut still lower. The hardness of 
the rock can make no permanent difference. This is well illus- 
trated even in so young and small a body as Lake Mendota 
at Madison, Wis., where the sandstone shelves southwest of 
Governor’s Island and Maple Bluff are cut to the same depth as 
the clay shelves west of Picnic Point and Second Point." 
If the shelf is being broadened at the same time by materials 
carried across and deposited on its front, there will be, between 
its upper surface and its steep front, a curve convex to the sky 
as shown above. This steeper slope begins, not at the depth 
where the power of the water ends, but at the depth at which 
the power of the water becomes insufficient to carry the entire 
load. From this depth the slope becomes progressively steeper 
to the depth at which the movement of the water is ineffective. 
Off the Atlantic coast of the United States, the depth at which 
the slope begins to steepen is usually fifty or sixty fathoms, but 
the maximum of steepness is not attained until a much greater 
depth is reached. The depth familiarly assigned to wave-base 
along this coast is one hundred fathoms, and this figure 
expresses fairly well the horizon at which the maximum steep- 
‘See hydrographic map issued by the Wisconsin Geological and Natural History 
Survey. 
