The St. Ignace plate. Relief from Copernicus GLO-30; rivers and lakes from USGS NHD; watersheds from the USGS Watershed Boundary Dataset; roads and city limits from Census TIGER 2024.
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St. Ignace’s center stands about 710 ft above sea level. Within three miles the land runs from 570 ft to 720 ft, a difference of 150 ft: gently rolling country.
Under the town center the state geologic map shows Mackinac Breccia (limestone, sedimentary-breccia), from the Early Devonian, 394 to 422 million years old.
The section below crosses 2 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 64×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Shale. Age: Silurian, 422 to 428 million years old. About 9 mi of the line.
Limestone, sedimentary-breccia. Age: Early Devonian, 394 to 422 million years old. About 4 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale.
Heights from Copernicus GLO-90 (a surface model: it includes buildings and tree canopy); summits from USGS GNIS; rivers from USGS NHDPlus HR; rock from the USGS State Geologic Map Compilation.
Rain that falls on St. Ignace runs straight into Lake Huron. From there it moves on through the Great Lakes, over Niagara Falls and down the St. Lawrence to the Atlantic.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of St. Ignace; river miles along the main stem, rounded.
