Wooldridge en the (River=Lake System of W. Mich. 145 
surface sand which overspreads the lower levels in their vicin- 
ity, The region bordering White lake on all sides is composed 
of clay, over spread with a stratum of sand varying, usually, 
from one foot to four feet in depth, though in places it is 
deeper. The clay surface reaches a hight of 15 to 30 feet above 
the level of the lakes, and a portion of the strip of land which 
separates it from lake Michigan, even, consists of a hill of drift 
material, mostly clay, partly covered with a timbered dune. 
I have also made observations which go to show a similar 
structure in the case of other lakes of this class, especially Duck 
lake, shown on the map, Pentwater lake, and Manistee lake. 
All these lakes seem to be submerged portions of the river- 
valleys in which they lie, and such I believe they are in fact. 
3rd. The present bottoms of these lakes are the product of 
sedimentation and not of erosion, and that to such an extent as 
to obliterate any features due to erosion which may originally 
have existed in them. Of this fact I have convinced ni} self by 
sounding through the ice on White lake and Duck lake, though 
I had anticipated a contrary result. 
4th. The original sIojdc of the bottoms of these lakes in the 
direction of drainage must have lieen much greater than of the 
country farther inland. I have myself sounded a depth of 84 
feet in White lake at the point indicated thus* on the ac- 
commpanying map. It is about seven miles from this point to the 
head of the marsh where the original lake bottom coincides 
with the present water level, from which it appears that a fall 
of least 84 feet existed in the original river bed between these 
points. From the head of the marsh up stream it is necessary to 
go at least twenty miles in order to find an equal change of level. 
5th. The valleys occupied by these lakes appear to have 
extended across the present border line of lake Michigan into 
deep water. At the foot of such river-lakes as these, in every 
instance known to the writer drifting sand-dunes now exist. 
At sometime not very remote a much more extensive system 
of drifting dunes existed in this region than is now foimd, but 
long reaches of these dunes have become timbered and fixed. 
In such places the dunes are separated from the present beach 
sand by an escarpment cut in the clay that imderlies (hem. 
This cla^' is what remains of the denuded surface of the older 
