186 Wisconsin Acadeniij of Sciences^ Arts and Letters-. 



of this till ridge, it swings around to its eastern side, and bearing away 

 from its southern end, comes to the Calumet river near Riverdale. Its ele- 

 vation at Summit is 40 feet; at "Washington Heights, 41 feet; but at sev- 

 eral points between these villages a survey made by the Chicago Drainage 

 Commission, reports its elevation as 35 feet above Lake Michigan. 



The best exposure of the structure of this portion of the beach is found at 

 Summit. An extensive excavation is made near the curving portion of 

 the beach, just as it is about to turn down the River Valley. The beds of 

 gravel dip toward the north with a low angle (10 — 15°). There is but little 

 sand with the gravel. For a short distance tbe excavation shows a depth 

 of 20 feet of beach deposits, but in passing eastward the pebbly clay which 

 underlies them rises perceptibly. The gravel is brown, stained to a depth 

 of 3 — 4 feet from the surface. Below this depth it is unstained. It is 

 but slightly calcareous, even in the finer sandy portions. The pebbles sel- 

 dom exceed one inch in diameter. They are worn smooth like those along 

 the present beach of the lake. 



We are told that shells of Unios and of smaller mollusks, also fragments 

 of wood, have been found at the base of the gravels, but none were at hand 

 at the time of our visit. Portions of the beach east of Blue Island are 

 quite sandy. 



South from the Calumet river this beach has a wide break, for the lake 

 discharged past the south end of Blue Island through the sag into the 

 Des Plaines river vaUey. The outlet from Blue Island, west to the point 

 where the sag narrows to pass through the Valparaiso moraine, has a 

 width of no less than three miles. Its north border is well marked in the 

 form of a sea cliff, on the north side of the sag, and on the south it washed 

 the foot of the Valparaiso moraine. An island locally known as Lane's 

 Island, lies in this outlet, and its borders are flanked by gravelly deposits, 

 to an elevation of about 35 feet above Lake Michigan. This island ap- 

 parently, was submerged at thfe time when the upper beach was formed, 

 for its surface is very level and has a coating of sand. Similarly, a large 

 tract of land west of Blue Island, which was not submerged at the time of 

 the formation of the Middle beach, was covered by the lake at the epoch 

 of the Upper beach. 



We were interested in noting a boulder-strewn belt occupying the por- 

 tion of the sag outlet, just east of the Valparaiso moraine. The boulders 

 in places, number several thousand per acre, so that it is very difficult to 

 till the soil. They are both Paleozoic and Archaen. They may, many of 

 them, have been dropped by ice-floes at the time of the lake expansion, 

 for the configuration of the channel west from here, is such as to impede 

 a free discharge into the Des Plaines valley, but a portion of them prob- 

 ably, were dropped there by the glacier, at a much earlier period, for other 

 gaps in the moraines, similar to the sag, where the lake had no outlet, 

 often have a large number of holders in them. • 



The lake at the epoch of the Middle beach, probably extended nearly as far 



