ABSTRACT The Indiana Dunes is a name commonly used for the eastern part of the Calumet Lacustrine Plain, generally referring to the large dunes along the coast from Gary, Indiana, eastward to the Michigan state line. However, the Calumet Lacustrine Plain also contains complex coastal landscapes associated with late Wisconsin to Holocene phases of ancestral Lake Michigan (e.g., mainland-attached beaches, barrier beaches, spits), including those formed during quasi-periodic decadal and shorter-term waterlevel variability that characterize modern Lake Michigan (e.g., beach ridges, dunes, interdunal wetlands). Major industrial development and other human activities have impacted the Calumet Lacustrine Plain, often altering these landscapes beyond recognition. Today, geological and paleoenvironmental data are sought to inform regional environmental restoration and management efforts and to increase the resiliency of the coastal landscape to ongoing disturbances. During this field trip, we will examine the relict shorelines and their associated nearshore and onshore features and deposits across the Indiana portion of the Calumet Lacustrine Plain. These features and deposits record the dynamic interaction between coastal processes of Lake Michigan, lake-level change, and long-term longshore sediment transport during the past 15,000 yr. Participants will examine the modern beach, the extensive beach-ridge record of the Tolleston Beach strandplain, a relict dune field, and the large dunes of the modern shoreline, including Mount Baldy. At Mount Baldy, we will focus on the landscape response to human modification of the shoreline. We will also explore the science behind dune decomposition chimneys—collapse features that caused a 6-yr-old boy to become buried more than 3.5 m below the dune surface in 2013 and highlighted a previously unrecognized geologic hazard.

ABSTRACT This field guide provides an updated synthesis of the stratigraphy and chronology of glacial deposits in central Indiana near the southern limit of glaciation in the midcontinent. Central Indiana contains evidence of multiple glaciations—deposits from the last two glaciations (Oxygen Isotope Stages [OIS] 2 [Wisconsin Episode] and 6 [Illinois Episode]) have been the focus of recent stratigraphic and chronologic investigation. New radiocarbon and optically stimulated luminescence (OSL) dating from outcrop and core has refined the timing of OIS 2 and OIS 6 ice sheet advances, outwash/slackwater aggradation, glacial lake formation, and eolian activity. Radiocarbon ages within or below late Wisconsin till from three sites within 5 km (3 mi) of the late Wisconsin maximum limit indicate an age of 24.0 k cal yr B.P. for maximum OIS 2 ice sheet extent in central Indiana, consistent with chronology from Illinois and Ohio. A subsequent >50 km (31 mi) readvance (21.6 k cal yr B.P.) across central Indiana came within 10 km (6 mi) of the maximum limit and in the western part of the field-trip area, terminated in glacial Lake Eminence. The start of outwash aggradation and associated slackwater sedimentation in the West Fork White River valley and tributaries began ca. 27 k cal yr B.P. and continued until ca. 20.5 k cal yr B.P., representing the timing of ice sheet advance into and out of the paleo–White River drainage basin. Ice sheet advance and retreat rates average ~40 m/yr before and after the global Last Glacial Maximum (ca. 26−21 k cal yr B.P.) when ice was within ~50 km of the late Wisconsin maximum. OSL dating of pre-Wisconsin outwash and glacio-lacustrine sediment return ages between ca. 218 and 127 ka, confirming deposition during OIS 6. These ages document spatially complex sedimentation in bedrock valleys beyond the Wisconsin limit.

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.