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Nearly 40 years ago, Bretz inferred that glacial Lake Chicago stood at the 189-m (620-ft) or Calumet level twice, first before the Two Creeks low-water phase and then again after the Two Creeks interval. Although Bretz argued for a double Calumet stage on theoretical grounds, he clearly attributed formation of the Calumet shoreline to the post-Two Creeks stage only. Willman, on the other hand, believed that Calumet shoreline features were formed during both Calumet stages. Eschman and associates argued that the Calumet phase was pre-Twocreekan in age, and that Lake Chicago did not return to the Calumet level following the Two Creeks interval. On the basis of available radiocarbon age control, we attribute formation of Calumet shoreline features and deposition of associated sediments in the type Calumet area at the south end of the Lake Michigan basin to a post-Two Creeks lake phase.

We have examined two areas of the Calumet shoreline in detail—the Rose Hill spit at Evanston, Illinois, and two sites on the Calumet beach at Liverpool, Indiana. Radiocarbon dates on nine wood samples from beneath and within the Rose Hill spit deposits range from 11,870 to 11,000 yr B.P. At the Liverpool East site, beach deposits contain driftwood dated at 12,400 and 11,740 yr B.P.; the older wood is possibly redeposited Glenwood material, and the younger is certainly no older than Two Creeks. Five additional dates, on wood and peat from an overlying thick organic layer and younger dune sand, range from 11,290 to 9,080 yr B.P., including a date of 9,920 yr B.P. on a tree trunk in growth position. At the Liverpool West site, the oldest date (11,815 yr B.P.) is Two Creeks, and two dates on material higher in the section fall within the range of dates from the Liverpool East site.

Although the radiocarbon evidence does not preclude the possibility of a pre-Two Creeks Calumet phase, the general lack of pre-Two Creeks dates from the Calumet beach indicates either that the pre-Two Creeks Calumet phase (Calumet I) was so brief that no prominent shoreline features were formed or that these landforms were obscured (destroyed?) during the post-Two Creeks Calumet phase (Calumet II). In any event, type Calumet shoreline features were not formed until the Lake Michigan Lobe readvanced to the Two Rivers Moraine in post-Two Creeks time.

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