96 BULLETIN OF WISCONSIN NATURAL HISTORY SOCIETY. VOL. 1, NO. 2. 
All these hills are without doubt of glacial origin. Their sur- 
faces are covered with boulders, mostly crystalline. Where cuts 
and other exposures have been made sufficient to show the internal 
structure, the aspect is always the same. There is an irregular 
accumulation of clay, sand, gravel and boulders of varying size. 
But surrounding these hills, filling the depressions between them 
and ascending their sides, there is the deposit of grayish clay ( i ) , 
with comparatively few pebbles and only an occasional boulder, 
which is indicated on the map. This deposit shows a fairly dis- 
tinct stratification, but considera1)le exposures of it are infrequent. 
The largest are those made by the brick yards in the Menomonee 
\'alley. About the- aquatic origin of this deposit there can be no 
doubt, nor about the fact that it is younger than the boulder clay. 
This being so, the interesting feature is that all the highest places 
of this neighborhood show the boulder clay as their surface de- 
posit. In some cases, the lower hills may have a lacustrine clay 
surface like the depressions. I have noticed no exposures show- 
ing this, but have suspected it from the scarcity of boulders. But 
without question all the higher hill tops are boulder clay. • 
The obvious conclusion to be drawn from this circumstance is, 
that at the time when the lacustrine clay was deposited, the 
boulder clay hill tops were islands, and the lower hills with a 
lacustrine clay surface, if such there are, were shoals. This fact 
explains the greater quantity of pebbles and the relatively large 
number of boulders in this formation, compared with the red 
clay to the eastward. The latter was deposited in open and deep 
water. Avhile the former was laid down in narrow seas, where 
there was a great deal of material washed down from the land, 
both by direct erosion and with the help of ice rafts. In some 
places, notably the ravines on the north side of the Menomonee 
A^alley, there appears to be a stratum of boulder clay overlying the 
lacustrine. This is easily explained by material rolling down the 
sides of the hill, either while the lower deposit was still forming 
or, more probably, after it had become dry land. 
The existence of an archipelago of boulder clay islands on the 
west side of ^lilwaukee and in the town of Wauwatosa during 
Champlain times is still further evidenced bv the existence of 
another formation, occupying a middle grovmd between the 
boulder clay and the lacustrine clay. In a large number of places, 
but invariably on the side of one of the hills, there are large ac- 
cumulations of sand and grav.el, w^hich are now often used as 
gravel pits. These accumulations have the evident ear-marks of 
(1) The term "clay" is used in this paper in its popular rather than its technical sense, mean- 
injr a fine-jrrained. cohesive earth. I do not know whether pure aluminous clay occurs in this 
neighborhood. Most of the clay, so-called is decidedly marly. 
