Geology and landforms

Summary

The archipelago is not just a set of named shores and wetlands. Its present landscape is a layered record of bedrock, glacial deposits, former lake levels, beach ridges, dunes, and wetlands. The collected Michigan map layers give the knowledge base a reproducible physical baseline without pretending that a regional polygon is a field observation.

What the collected maps show

The project intersected the collected geology layers with the current OSM island polygons and calculated areas in EPSG:3078. In the Quaternary layer, 116.789 km² is labelled lacustrine sand and gravel, 63.912 km² dune sand, and 11.733 km² coarse-textured glacial till. These are map-class areas, not mutually exclusive claims about every parcel’s surface material.

The bedrock layer contains four labels within the island intersection: Garden Island Formation, Bois Blanc Formation, Detroit River Group, and Midcontinent Rift. The collected glacial-landsystems layer adds the broad classes lacustrine coarse, bedrock, lacustrine fine, and proglacial outwash. The state Critical Dune Areas layer contributes four barrier-dune polygons totalling approximately 1,642 acres. Its regulatory purpose means it should not be mistaken for a complete inventory of every dune or beach ridge.

Lake levels and shorelines

Regional Great Lakes geology provides the interpretive frame. Leverett and Taylor’s 1915 synthesis maps the major ancestral lake shorelines and the isostatic tilt of the basin. Thompson and Argyilan’s 2020 synthesis explains that beach ridges preserve the elevations of former highstands and that lake levels fluctuate on both shorter and longer cycles. The collected EGLE linear layer contains nine features labelled SHORELINE, but neither that layer nor the regional publications supplies a dated, island-by-island chronology here.

That distinction matters for Beaver Island history. A beach ridge, dune, bluff, or soil series can be evidence of a physical setting; it cannot be assigned to the Nipissing, Algoma, or another lake stage merely because the regional stage exists. The specific ridge, its elevation, and its age still require field or published island evidence.

Connections to human history

The geology constrains the historical themes already in the graph. Lacustrine and dune deposits help explain the island’s beaches, wetlands, and agricultural limits; the forest and lumbering article depends on the reconstructed soils and land cover; and reefs, shoals, and shorelines shaped both fishing grounds and navigation. Those are relationships of setting and constraint, not claims that the collected map layers alone explain settlement or economic change.

Open questions

• Which mapped beach ridges and dune complexes correspond to named places such as Donegal Bay, Martins Bluff, and Beaver Island Public Beach?
• Can elevations, cores, or published dates connect any Beaver Island ridge to a specific Lake Michigan highstand?
• Which bedrock units are exposed at the shore, and which are covered by Quaternary deposits?
• What island-specific field studies, soil profiles, or tribal and archaeological sources can be cited without disclosing protected cultural-site locations?