Archipelago physical geology and hydrogeologic context crosswalk, 2026

What this adds

The geology retrieval pipeline already held more physical context than the geology theme exposed: mapped glacial-drift aquifer character, Quaternary surface materials and shoreline features, glacial-land-system classes, regulatory Critical Dune Areas, a bedrock fault layer, a glacial-lobe boundary, and USGS State Geologic Map Compilation (SGMC) lithology and contact layers. This crosswalk makes their island intersections searchable while keeping regional map products separate from soils, wetlands, forest stands, and field observations.

The most consistent result is the state aquifer layer. All seven named islands intersect its class-IV mapped glacial-drift area, described by the source as drift that may be an unconfined aquifer to bedrock, with delineation not possible from available data. That wording is a screening classification, not evidence of a usable well, a groundwater flow path, or a parcel-level water source.

Quaternary, glacial-land-system, and regulated-dune frame

The expanded crosswalk now carries the other held physical layers into the same seven-island geometry. This makes the relationship among mapped surface material, broad glacial setting, shoreline-feature coverage, and regulated dunes explicit:

These are calculated intersections with the held OSM island geometries. The Whiskey zeros and the zero shoreline rows are coverage results for the particular generalized layers, not proof that an island has no Quaternary deposit, shoreline feature, glacial setting, or dune. Critical Dune Areas are a regulatory subset: they identify mapped state-regulated polygons, not every natural dune, beach ridge, or interdunal landform. The Quaternary and glacial-land-system classes are broad regional map products and should be read alongside SSURGO, GLO field observations, MNFI site descriptions, and mapped dune reports rather than as parcel or occurrence boundaries.

Glacial-drift aquifer context

The held aquifer snapshot contains 21 source polygons. The island intersections are:

The areas are calculated intersections with OSM island geometries, not official aquifer acreage. They should be compared with SSURGO map units, NWI wetlands, inland-water records, and site-specific well or hydrogeological evidence. A mapped aquifer class does not establish drinking-water quality, recharge, depth to bedrock, groundwater-surface-water connectivity, or contamination risk.

Bedrock units in the USGS regional layer

The SGMC lithology polygons intersect the named-island frame as follows. Water is reported because it is an SGMC category, but it is not a geologic formation.

The Whiskey and Fox results must not be interpreted as proof that those islands lack bedrock or have no geologic unit. They are a limitation of the generalized SGMC polygons and the working intersection frame. The separate Michigan EGLE bedrock layer and the USDA/SSURGO and Quaternary layers provide different map products and scales; none should be silently substituted for another.

The resolved unit vocabulary is useful for connecting the regional bedrock map to the existing physical narrative: Bois Blanc is mapped as Early Devonian limestone with minor dolostone; Detroit River Group as Middle Devonian dolostone with minor limestone and incidental sandstone; and Point aux Chenes as Late Silurian shale. These are generalized source descriptions, not observations of exposed shoreline rock at every island location.

Contacts, faults, and glacial boundaries

The SGMC contact layer intersects six of the seven island frames. Calculated intersected lengths are:

The held Michigan bedrock-linear-feature layer contains one fault in the broad AOI, but it does not intersect any named-island geometry. The held glacial-lobe layer contains one Michigan/Saginaw interlobate boundary, also with no named- island intersection. These zeroes are useful coverage results: the layers do not put a mapped fault or lobe boundary inside the current island frame. They are not evidence that no structural lineament or glacial boundary is relevant to island formation, because regional generalized boundaries, offshore features, and different map scales may not coincide with island polygons.

How this connects to the ecology graph

This crosswalk supplies physical context for the other evidence layers:

• SSURGO supplies island-scale soil map units and the 1974 survey supplies narrative links among dunes, beaches, depressions, drainage, and woodland;
• NWI and EGLE wetland layers supply modern mapped wetland/deepwater and hydric- soil classes;
• circa-1800 GLO reconstruction supplies historical vegetation classes;
• MiFI supplies current public-inventory stand and covertype polygons for Beaver Island; and
• MNFI supplies field-surveyed natural communities, trees, water regimes, and habitat-linked wildlife observations; the wildlife and bird indexes keep named species, survey classes, and island-management records separate from these physical polygons.

The correct interpretation is a layered physical-to-biological evidence chain, not a single geology-to-habitat model. Shared spatial overlap can identify a research candidate; it cannot establish causation, historical persistence, habitat suitability, or species occupancy without source-specific field evidence.

Evidence limits

All measurements use the held OSM named-island geometries and equal-area EPSG:6933 calculations. The source layers are regional products with different dates, resolutions, classification schemes, and offshore coverage. The SGMC water category is excluded from geological interpretation, and unresolved or null contact attributes are retained rather than invented. The crosswalk does not infer exposed bedrock, groundwater availability, aquifer quality, glacial event chronology, legal wetland status, or parcel conditions.

Added 3 August 2026. Recent changes