Page:The American journal of science, series 3, volume 49.djvu/24

6 6 W. Upham — Champlain Subsidence and

beach at 30 feet was formed after that at 15 feet is shown by the occurrence in some places of a peat deposit, described by Andrews and Leverett, which passes underneath the 30 feet beach and is continuous from its upper side down to the lower beach. The peat marks a land surface over which the lake rose to form the middle or third beach, after having stood at the lower or second beach for some time. Still later, however, it probably again stood at the lower level, corre- sponding to the present watershed in the abandoned outlet. This old channel of outflow, at its summit, as I am informed by Mr. Ossian Guthrie from the canal survey, is now 11 feet above the mean level of Lake Michigan, but the surface there is postglacial silt ; at another point, where the channel bed consists of till, and at a third place where the bed is rock, its height in each case is only eight feet above the present lake, or 590 feet above the sea. The mouth of Lake Warren appears to have been at first near Lemont, on the Des Plaines river about 25 miles from the lake, where the river valley was obstructed by drift which suffered erosion, allowing the mouth of the lake to be transferred gradually upstream, at the same time being lowered, to its final position ten miles from the lake shore in Chicago. Epeirogenic movements, between the times of formation of the second and third beaches, slightly lifted the outlet and adjacent portion of the course of the Des Plaines river, as compared with the southern and southwestern part of the Lake Michigan basin, causing the old lake to extend a little farther on that side than before. Toward the north and east, however, this change was doubt- less more than counteracted by the rapid differential rise of the land.

Fresh-water shells are found abundant in the 15 feet beach at Evanston and elsewhere southward through Chicago. All the species obtained, representing ten or more genera, are still living in this region. Wood of oak and cedar, and the thigh bone of a deer, have been also found in the same beach at Evanston.*

For the distance of about 185 miles from Chicago north to the south end of Green bay, the highest shore of Lake Warren appears to be now nearly level, for Mr. Taylor finds evidence of submergence only to a height of some 20 feet above that part of Green bay and the neighboring lake shore. Thence northward, however, the beach rises about 14 feet per mile for 110 miles to Cook's Hill, near the north end of this bay ; in 60 miles from that latitude north to Houghton, it has an ascent of 260 feet, or 4-J feet per mile ; but in about 90

" Eaised Beaches of Lake Michigan," before cited, p. 189.
 * H. M. Bannister, Geology of Illinois, vol. iii, 1868, pp. 241, 242. F. Leverett,