Abstract The Holocene began about 10.7 ka following the glacial readvance of the Younger Dryas (Valders) interval that terminated the Pleistocene. World sea level had fallen to about -54 m. Nearly half the present United States continental shelf was then a coasta] plain with vegetation ranging in nature from subarctic tundra to coniferous woodland. A fluctuating transgression ("Flandrian" stage) followed, accompanied by rising temperatures. Reaching the present coastline about 6 ka, the transgressing seas drowned river valleys, creating estuaries and dendritic embayments. As barrier spits and islands developed, the estuaries and embayments became lagoons. During the later Holocene, world sea level was modulated by numerous negative fluctuations, signa Jing cool intervals that are indicated by pollen analyses and neoglacial advances. Extra-warm cycles were characterized by higher storm frequency; in areas favorable for preservation, these conditions are recorded by distinctive sets of beach ridges. Local paleogeography was dominated in most places by a tectonic “groundswell,” such as is presently found throughout the collapsing forebulge regions along subducting plate margins of southern Alaska and on the taphrogenic coast of California. More or less stable platforms are found only in northwest Alaska and Florida. Present coasts are widely affected by the post-Little Ice Age warming, which has led to steric and glacio-eustatic sea-level rise. The warming has also slowed the velocity of the Gulf Stream and other geostrophic currents, reducing the Coriolis-controlled dynamic tilt and causing further sea-level rise along the mainland coasts.

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).

The names of the Huron, Erie, and Ontario Lobes imply that glaciers followed these lake depressions, but the flow patterns of these lobes were complex and changed several times during the Wisconsin glaciation. The sublobes of the southwestern part of the so-called Erie Lobe were more often an extension of the ice coming down Huron Basin, and ice from both basins participated in these sublobes. A review of the studies of Wisconsin-age deposits in the area of the three lobes indicates the emphasis that has been placed on investigations of tills by multiple methods, on paleontological studies of the interstadial and late-glacial deposits, and on radiocarbon dating. A threefold time-stratigraphic division of the Wisconsin Stage in this area is based upon synchronous fluctuations by several glacial lobes, and upon climatologic inferences from paleontologic studies. Early and late Wisconsin experienced maximum glacial advances; middle Wisconsin was dominated by interstadial retreats. The first Wisconsin glacial advance reached into the St. Lawrence Lowland only, and was followed by a glacial retreat during the St. Pierre Interstade about 65,000 radiocarbon yrs B.P. The second major glacial advance (by several lobes) went farther, but did not reach as far south as the late Wisconsin glaciation in Indiana and Ohio. In Pennsylvania and New York it was more extensive than the late Wisconsin, if the Olean Drift is indeed of early Wisconsin age. The source of ice was centered in the eastern Canadian Laurentide area. Mid-Wisconsin glacier margins retreated several times far into the Huron and Ontario Basins, or even north of them. Three main retreats were probably interrupted by two readvances which reached into the Erie Basin but not south of the Lake Erie watershed. Mid-Wisconsin time began more than 50,000 yrs B.P. and ended about 23,000 yrs B.P. During late Wisconsin time the major glacial advance in the western part of the region investigated reached its farthest extent south in at least three pulses: 21,000, 19,500, and 18,000 yrs ago. The oscillating retreats were interrupted by three documented readvances: about 17,000, 15,000, and 13,000 yrs B.P. The source of ice was primarily in the western Laurentide center. No evidence has been found here for the Valders readvance which took place in the Lake Michigan Lobe.