ABSTRACT This field guide discusses the dune types and processes, ecology, and geomorphic history of the largest freshwater dune systems on the southeastern shore of Lake Michigan. From north to south, stops include P.J. Hoffmaster State Park, Gilligan Lake/Green Mountain Beach Dune, Saugatuck Harbor Natural Area, and Grand Mere and Warren Dunes State Parks, Michigan. All of the sites are low, perched transgressive dune complexes. Moving from the lake inland, the typical dune complex in this area consists of incipient foredunes, an established foredune ridge, a parabolic dune complex, and a back-dune ridge complex. All stages of ecological succession are typically present in the larger dune complexes. Surface changes in Lake Michigan dunes are driven by spatial gradients in sand flux, which, in turn, are determined by a complex interaction among wind dynamics, vegetation patterns, and preexisting topography. Surface change patterns are modified by seasonal effects, with the majority of sand transport being associated with strong storms in the autumn, winter, and early spring. Sand can be temporarily stored in niveolian deposits during the winter, leading to oversteepened slopes, which collapse during the spring thaw. Current dune complexes largely formed during and after the rise in lake levels to the Nipissing high lake level, ca. 4.5 ka. Broad fields of relatively low dunes developed during the lake-level drop following the Nipissing high. Beginning with the rise to the Algoma high lake level, ca. 3.2 ka, the lakeward edges of these fields were episodically reworked, forming large parabolic dune complexes. A period of widespread dune stability formed the Holland Paleosol, a spodic inceptisol. Dune growth and migration resumed prior to European settlement of the area and continues today. Foredune complexes grow wider and higher during periods of low lake levels, but narrow during periods of high lake level due to scarping at their lakeward edges.

The Nipissing phase of ancestral Lakes Michigan, Huron, and Superior was the last pre-modern highstand of the upper Great Lakes. Reconstructions of past lake-level change and glacial isostatic adjustment (GIA), as well as activation and abandonment of outlets, is dependent on an understanding of the elevation of the lake at each outlet. More than 100 years of study has established the gross elevation of the Nipissing phase at each outlet, but the mixing of geomorphic and sedimentologic data has produced interpreted outlet elevations varying by at least several meters. Vibracore facies, optically stimulated luminescence and radiocarbon age control, and ground-penetrating radar transects from new and published studies were collected to determine peak Nipissing water-level elevations for the Port Huron (Lake Huron), Chicago (Lake Michigan), and Sault (Lake Superior) outlets. Contemporary elevations are 183.3, 182.1, and 195.7 m (International Great Lakes Datum of 1985 [IGLD85]), respectively. These data and published relative hydrographs were combined to produce one residual hydrograph for the Port Huron outlet that best defines the rise, peak, and rapid fall of the Nipissing phase from 6000–3500 calendar years ago. Establishing accurate elevations at the only present-day unregulated outlet of the Great Lakes and the only ancient outlet that has played a critical role in draining the upper Great Lakes since the middle Holocene is a critical step to better understand GIA and water-level change geologically and historically. The geologic context may provide the insight required for water managers to make informed decisions to best manage the largest freshwater system in the world.

ABSTRACT This field guide explores the geomorphology, ecology, contemporary processes, sedimentary structures, and geomorphic history of the large freshwater dune systems on the southeastern shore of Lake Michigan. Recent research studies on varying aspects of the dunes are highlighted at each stop. From north to south, these stops include P.J. Hoffmaster State Park near Muskegon, Michigan; Gilligan Lake and Green Mountain Beach southwest of Holland, Michigan; Saugatuck Dunes State Park and Saugatuck Harbor Natural Area, both near Saugatuck, Michigan; Warren Dunes State Park and Grand Mere State Park between the Indiana–Michigan border and Benton Harbor, Michigan; and Mount Baldy on the eastern edge of the Indiana Dunes National Lakeshore, Indiana. All of the complexes described are low perched transgressive dune complexes that are migrating inland over former lake plains or baymouth bars. Moving from the lake inland, the typical dune complex in this area consists of incipient foredunes, an established foredune ridge, a parabolic dune complex, and a back-dune ridge complex. All stages of ecological succession—beginning with a pioneer community dominated by beach grasses and ending with a mesic forest dominated by oak, maple, and beech—are typically present in the larger dune complexes. Like coastal dunes everywhere, surface changes in Lake Michigan dunes are driven by spatial gradients in sand flux, which, in turn, are determined by a complex interaction among wind, vegetation patterns, and preexisting topography. The patterns of surface change are modified by seasonal effects, with the majority of sand transport being associated with strong storms in the autumn, winter, and early spring. Sand can be temporarily stored in niveolian deposits during the winter, leading to oversteepened slopes, which collapse during the spring thaw. A variety of sedimentary bed forms and structures can be viewed in dunes along the southeastern shore of Lake Michigan, including wind ripples, lag deposits, raindrop impressions, adhesion ripples, adhesion warts, eolian turrets, sand pedestals, surface patches of fine-grained dark sand, pinstripes, paleosols, cross-bedding, climbing ripple lamination, niveolian deposits, and avalanche lobes. Most of these features are best seen immediately after strong storms in the autumn and winter. Remnants of older dune surfaces are exposed in a few places in back-dune ridge complexes; however, the current dune complexes are largely a product of events that occurred during and after the rise in lake levels to the Nipissing peak (ca. 4.5 ka). Broad fields of relatively low dunes developed during the drop in lake levels following the Nipissing peak. Beginning with the rise to the Algoma high lake level (ca. 3.2 ka), the lakeward edges of these fields were episodically reworked, forming the large parabolic dune complexes. A period of widespread dune stability resulted in the development of the Holland Paleosol, a particularly well-developed paleosol with Spodosol characteristics. Widespread dune growth and migration resumed prior to European settlement of the area and continue today.

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.