XI 
The boulder clay, which lies over the solid rock, has great thickness beneath 
the waters of the lake, and on the shore of the lake northward at Glencoe, it 
rises 75 feet above the surface of the water. This clay is irregularly bedded, 
but in a general way three layers may be distinguished; first and lowest, a 
quicksand; second, “permeable” boulder clay well stratified; third, “wet” 
boulder clay obscurely stratified. In this last bed, the tunnels for obtaining 
water for the city have been excavated. 
In the vicinity of Michigan City, Indiana, a boring has shown the bed rock to 
be about 300 feet below the surface of the lake, covered with 143 feet of quicksand, 
the boulder clay lying over the quicksand. The clay contains small angular boul- 
ders, which are frequently glaciated. The upper part contains pockets of gravel the 
origin of which has been much discussed. One hypothesis is that the gravel now 
forming the pocket, was a frozen mass at the time of the deposit, and that it has 
behaved as a rock would have done under the circumstances, and become imbed- 
ded in the clay. The other suggestion is that the gravel was washed from the 
surface of the glacier into crevasses or wells by the surface waters and formed 
into a conical pile which became covered with finer sediment. In the section of 
the bluff at Evanston, represented in figure 2, above the shingle of the lake shore 
is seen a vertical section of clay. The black point in the diagram of the clay, 
represents one of these gravel pockets as it was observed. It was fifteen inches 
in diameter, and extended four feet nearly horizontally in the clay. The la- 
minae of the clay were contorted, and seemed to have the pocket of gravel for 
an axis. 
It is in the fragments of Devonian shales contained in these upper clays, that 
occur the macrospores of Protosalvinia chicagoensis discovered and described by 
B. W. Thomas and Dr. H. A. Johnson. 
In the northern part of the country the clay lies very thick over the rock, but 
in the region immediately about Chicago excepting lake ward, it varies from none 
at all to a few feet in thickness. The clay at the surface is usually covered 
with a few inches and sometimes a foot or two of mould and soil, and in level 
areas and slight valleys, it is covered with swamp and peat. 
Blue Island ridge and the “Sag Timber Hills” marked B and C on the map, 
is a mass of boulder drift lying above the blue clay. It is composed of large 
boulders, coarse gravel, and sand imperfectly stratified. Blue Island station at 
the foot of this ridge is 22 feet above Chicago datum, but the highest point 
in Blue Island Village which is near it and upon the ridge is 62.5 feet above 
datum. The highest point on the ridge is 89.2 feet. The surface of the 
“Timber Hills” is very undulating. The highest point is near the west- 
ern margin, and is 140 feet above datum. There is a detached mass lying 
south of the canal feeder called Lane’s Island with a small lake on its top. Sim- 
lar hills of drift exist at Lombard just beyond the margin of the county. 
Chicago occupies a part of an ancient bay of Lake Michigan. At the time 
the bay existed, there was an outlet of the lake in the southwest part of the 
county through what is now the Des Plaines river. The margin of this ancient 
bay is now marked by shore lines, or beaches. There are three distinct beaches 
marking three distinct stages in the waters of the lake. They may be designated 
as the Upper beach, the Middle beach and the Lower beach. The Upper beach 
is the older, the Middle beach the next in age and the Lower beach is a recent 
beach. The altitude of the Upper beach, which is the farthest from the lake, is 
about 55 feet above the lake, or six hundred thirty-seven feet above the seat 
The water line of the Middle beach is about thirty feet above the lake. The Ke- 
