The Cohasset 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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Cohasset’s center stands about 2,850 ft above sea level. Within three miles the land runs from 1,290 ft to 3,500 ft, a difference of 2,210 ft: mountain country. The highest named summit within 15 miles is Bald Mountain, about 5,780 ft, 13 miles east.
Under the town center the state geologic map shows Tertiary pyroclastic and volcanic mudflow deposits, unit 9 (Cascade Range) (andesite), from the Late Oligocene to Pleistocene, 2 to 24 million years old.
The section below crosses 6 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 4×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Andesite. Age: Late Oligocene to Pleistocene, 2 to 24 million years old. About 13 mi of the line.
Mafic-volcanic. Age: Permian to Jurassic, 145 to 299 million years old. About 4 mi of the line.
Sandstone, conglomerate. Age: Miocene to Pleistocene, less than 23 million years old. About 4 mi of the line.
Andesite, basalt. Age: Late Oligocene to Pleistocene, 2 to 24 million years old. About 2 mi of the line.
Plutonic, gneiss. Age: Paleoproterozoic to Late Cretaceous, 66 to 2,500 million years old. About 0.9 mi of the line.
Mafic-volcanic, greenstone. Age: Paleozoic to Mesozoic, 66 to 542 million years old. About 0.7 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.
A raindrop that lands in Cohasset reaches San Francisco Bay and the Pacific Ocean by way of the Anderson Fork, Mud Creek and the Sacramento: about 210 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Cohasset; river miles along the main stem, rounded.
