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.

The types and spatial distribution of subsurface sedimentary deposits in the Calumet and Toleston Beaches of ancestral Lake Michigan were studied to better understand the evolution of the southeastern shore of Lake Michigan. Deposits of eight depositional environments were recognized: (1) dune, (2) foreshore, (3) upper shoreface, (4) lower shoreface, (5) offshore, (6) back-barrier lacustrine, (7) paludal, and (8) glacigenic. The Calumet Beach formed at the end of a rise in lake level following the Two Creeks phase, a time period of low lake level in the Lake Michigan basin, to the Calumet level. This trasgressive event was primarily erosional and produced a ravinement throughout the study area. Locally, however, relief on the underlying till of the Lake Border Moraine was instrumental in the preservation of nearshore sediments. Progradation of the Calumet shoreline produced a vertical stacking of shallow-water coastal sediments over deeper water deposits. Lakeward translation of the shoreline occurred for an unknown period of time, until the altitude of the lake dropped to the level of the Chippewa phase of ancestral Lake Michigan. Unlike the transgression from the Two Creeks level to the Calumet level, the post-Chippewa transgression to the Nipissing I level was dominantly depositional. This transgressive event is recorded in an ascending sequence of back-barrier lacustrine, dune, and foreshore deposits in the western part of the study area and by the onlap of the toe of the Calumet dune and beach complex by back-barrier lacustrine, palustrine, and dune sediments.

A succession of molluscan faunas preserved in the upper 366 cm of a vibracore recovered from the Cowles Bog area records a series of long- and short-term environmental changes in the Lake Michigan basin since the low-water Lake Chippewa phase. The rise in the water to the Nipissing I level during the post-Lake Chippewa transgression affected the water table landward from the lake and brought into existence a small lake or pond at the core site. This event is recorded in the core at a depth of about 366 cm by a change in lithology from interbedded organic layers and fine sand to a fossiliferous, calcareous micrite (marl) and by the appearance of molluscs, in which the overwhelming preponderance of taxa and individuals prefer unpolluted, well-oxygenated perennial aquatic habitats. This molluscan assemblage continues to dominate the core between 366 cm and 190 cm. Between 182 cm and 122 cm, this association of species is almost completely replaced by taxa tolerant of water bodies that are becoming filled with organic debris, by species that can inhabit temporary water bodies, and by marsh-inhabiting terrestrial species. Paludification of the site, from 122 cm to the surface, is indicated by an assemblage dominated by terrestrial species that prefer wet to very moist substrates. These occur in association with pisidiid clams and aquatic gastropods capable of living in seasonal bodies of water. Short-term climatic events are suggested by rapid increases in the abundance of aquatic taxa in the upper 100 cm of the core. These faunal changes are interpreted as responses to a rise in the local water table due to increased precipitation.

Four late Wisconsinan stratigraphie units crop out in coastal bluffs of Lake Michigan, northern Illinois. The lowermost unit is a muddy, matrix-supported, stratified diamicton exhibiting abundant sedimentary structures. Stratification is manifested by silt laminations, clay beds, layers of fractional gravel, and coarse-tail grading. Some beds are deformed by loading or folding. Intraformational units have concave erosional contacts with variable dip directions and slopes. Pebble fabrics are random or weakly developed, with wide dispersion of pebbles about the mean axes. These characteristics of the lower diamicton are consistent with resedimentation by subaqueous sediment gravity flows. The lower diamicton is overlain by a coarsening-upward sequence of lacustrine silt and clay and proglacial deltaic and fluvial sediment. This unit was deformed by an ice advance that deposited lodgement till characterized by massive structure, uniform grain size distribution, and well-developed pebble fabrics with most elongate pebbles aligned parallel to ice flow. Subglacial meltwater eroded channels into underlying deformed lacustrine sediment and deposited stratified sand. The entire sequence is overlain by lacustrine sediment deposited in glacial Lake Chicago following retreat of the ice margin northward into the Lake Michigan basin.

X-ray diffraction analyses of till samples collected from multi-till exposures along the Lake Michigan shoreline in Michigan and Wisconsin show a systematic variation in the clay-mineral composition of successive till sheets. A general increase in the relative amount of 10Â clay occurs within a succession of till sheets, beginning with early or middle Wisconsinan Glenn Shores till and continuing through late Wisconsinan (Woodfordian) Ganges–New Berlin till and Saugatuck–Oak Creek till. A significant decrease in the relative amount of 10Â clay, however, occurs within the post–Mackinaw Interstade (late Woodfordian) Ozaukee-Haven and Two Rivers tills. These changes in clay-mineral composition are apparently related to major ice-margin fluctuations since a significant retreat of the Lake Michigan Lobe has been recognized between deposition of each of the above till sheets. Morphostratigraphic correlation of the Powell Moraine of the Erie Lobe with moraines of the Saginaw and Lake Michigan Lobes, as well as correlation between till units of the Saginaw and Lake Michigan Lobes, indicates that the retreat recorded between deposition of Ganges–New Berlin and Saugatuck–Oak Creek tills of the Lake Michigan Lobe is correlative with the Erie Interstade.

Three radiocarbon dates on wood, including one on a log from the type section of the Two Rivers till, show that the age of this till unit is unquestionably Greatlakean (post-Twocreekan). The Two Rivers till, now formally designated the “Two Rivers Member of the Kewaunee Formation,” was named in 1973 by Evenson for fine-grained reddish-brown till found along the Lake Michigan shore north of Two Rivers, Wisconsin. The till was correlated with till of similar lithology that overlies the Two Creeks Forest Bed at the Two Creeks type section, and thus the till was considered post-Twocreekan (Greatlakean) in age. Unlike the age of the Valders till, which has been hotly debated (whether pre-Twocreekan or post-Twocreekan) during the past 15 years, the age of the Two Rivers till has not been the subject of direct controversy. However, the age of the Two Rivers till at its type locality has not previously been demonstrated by radiometrically dated material. Part of a large log enclosed in till was collected from the Two Rivers type section in 1968, about three years before Evenson began his investigations in the Twin Rivers lowland, but the existence of this sample remained generally unknown. The wood has now been dated at 11,910 ± 120 yr B.P. (ISGS-1058), thus proving that the till is younger than the Two Creeks Forest Bed from which the log must have been derived by the ice. Two additional dates, from a site on the south side of Kewaunee, also serve as confirming dates for the Greatlakean age of the Two Rivers till. Wood from a black, snail-rich peat layer has been dated at 11,700 ± 110 (ISGS-1061) and 11,650 ± 170 (ISGS-1234) yr B.P. The organic layer underlies fine-grained reddish-brown till that has been correlated with similar till that overlies the Two Creeks Forest Bed at its type section and thus was called Two Rivers till by Acomb and others (1982).

A Twocreekan organic horizon, which is underlain by till of the Haven Member and overlain by till of the Two Rivers Member of the Kewaunee Formation, was investigated near Kewaunee, Wisconsin. Wood from this horizon was dated at 11,700 ± 110 B.P. (ISGS-1061) and 11,650 ± 170 B.P. (ISGS-1234). The insect fauna from the Kewaunee site has many elements in common with the insects from the type section of the Two Creeks Forest Bed, 14 km to the south. These include the northwestern carabid Asaphidion yukonense , northern carabids Carabus taedatus and Bembidion grapii , and the northern staphylinid Acidota quad rata . In contrast, the Kewaunee site fauna appears to have inhabited a somewhat colder environment, as suggested by the occurrence of the carabids Cymindis unicolor and Pterostichus (Cryobius) spp. We interpret the Kewaunee specimens of aquatic, water-marginal, and upland species to represent an allochthonous rather than an autochthonous assemblage.

A complex of abandoned Lake Michigan beach ridges at Baileys Harbor, Wisconsin was investigated to establish the type, onset, and periodicity of ridge-forming processes. A further objective of the study was to place the development of the complex into the context of the postglacial history of the Great Lakes region. Surface profiles were constructed, and samples of sediment and peat were collected and analyzed. Results of pollen and radiocarbon analyses were used to infer the kind and timing of climatic conditions affecting lake levels, ridge accretion, and peat accumulation. A suggestion is made for an asymmetrical rate of change between high and low lake levels. The ridges accreted during four episodes of low or falling lake levels separated by three periods of high or rising water, during which erosion of earlier ridges occurred. Peat first began to accumulate in the interridge swales after the first erosional event, and construction of a truncating ridge began not long before 1,000 B.P. A pollen core extracted from behind the oldest ridge revealed a vegetational sequence that closely corresponds with post-Algoma lake-level fluctuations reported by other workers for the Lake Michigan basin.

The various elevations at which the lakes in the Michigan and Huron basins stood during the Holocene Epoch were previously thought to be the result of isostatic uplift of the northern outlets and discontinuous erosion of the southern outlets. Lake-level fluctuations of smaller magnitude, however, were caused by climatic changes. Correlation of climatic variables and lake-level variations over the past 100 years indicate that high lake levels occurred during cold periods when arctic air frequently converged over the Great Lakes with warm moist air from the Gulf of Mexico, causing increased rainfall. At the same time, increased cloud cover decreased evaporation. In addition, lower temperatures caused more winter precipitation to be held in storage in the basin as snow and ice, later to be made available for run-off during the spring. Major periods of climatically induced high lake levels can be recognized where sediments along the beach were eroded during the high levels, and in areas behind the beach where streams aggraded, active marshes formed, and soils formed on eolian sands. Radiocarbon dates of organic material that accumulated in these areas during high-water periods, coupled with analysis of the historic record of lake-level fluctuations, indicate that water level has varied cyclically, with a short 100- to 150-year cycle superimposed on a longer 400- to 500-year one.

Strandlines and related features representing former high glacial-lake levels possibly related to the Glenwood and Calumet phases of glacial Lake Chicago were identified from Oceana County north to Benzie County in the northwestern lower peninsula of Michigan. Lacustrine features occur as far as 25 km inland from the Lake Michigan shore at altitudes above the rebounded water planes of glacial Lake Algonquin and the Nipissing Great Lakes identified by earlier investigators. In order to determine if the water planes identified in this study and those of previous investigators represent the Glenwood and/or Calumet phases, water-plane altitudes are compared with a height/distance curve for the highest level of the Lake Algonquin water plane constructed by J. W. Goldthwait. The Goldthwait curve north of his zero isobase indicates the nature of glacial isostasy for the northern Lake Michigan basin following the development of the Glenwood and Calumet phases. The rebounded water planes of both the Glenwood and Calumet phases should follow exponential curves similar to that of Lake Algonquin but at higher altitudes. The water-plane data were projected onto a vertical plane oriented perpendicular to Goldthwait’s Lake Algonquin isobases in the northern part of the Lake Michigan basin and parallel to the axis of Lake Michigan in the southern part of the basin. From Ludington north to Frankfort, Michigan, the array of water-plane altitudes is diffuse but has an upper boundary that corresponds to a theoretical Glenwood II water plane. Only two sites occur at elevations high enough to be attributed to a Glenwood I water plane. Lacustrine features that occur at lower altitudes within the array, but above the Algonquin level, are within range of a theoretical Calumet water plane. Correlation of water-plane data with either the Glenwood or Calumet phase will probably remain unclear until ages are determined for many of the features.

A numerical model of a spherical viscoelastic self-gravitating Earth has been used to predict the glacio-isostatic deformation of the Lake Michigan basin during late-glacial and postglacial times. Predictions of present rate of vertical movement agree well in trend but slightly exceed in magnitude the observed rate of tilting indicated by lake-level gauges. Predicted uplift curves for the four dominant outlets controlling the ancestral lakes of the Lake Michigan basin indicate an outlet chronology comparable to that proposed by glacial geologists despite the fact that the Chicago and Port Huron outlets are not predicted to be stable as is commonly believed. Predictions of tilting of the Algonquin shoreline match observations north of the Algonquin hinge line, but the predicted shoreline plunges below the present level of Lake Michigan at the hinge line location. In opposition to the commonly held belief in crustal stability south of the Algonquin hinge line, the predictions indicate considerable vertical movement there continuing to the present. If the predictions are correct, the subhorizontal shorelines south of the hinge line have been misinterpreted because the Glenwood shoreline, reported to be subhorizontal there, is predicted to be strongly tilted. Alternatively, correct interpretation of this shoreline implies serious deficiencies in the assumed ice-sheet history or Earth rheology used as input to the model.