The Twain 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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−3° on Nov. 12, 1985, the coldest reading for this week of the year since 1895. Next is 5° in 1939.
Twain’s center stands about 2,900 ft above sea level. Within three miles the land runs from 2,780 ft to 5,390 ft, a difference of 2,610 ft: mountain country. The highest named summit within 15 miles is Grizzly Peak, about 7,660 ft, 14 miles east.
Under the town center the state geologic map shows Paleozoic marine rocks, undivided, unit 4 (Western Sierra Nevada) (argillite, quartzite, phyllite), from the Ordovician to Triassic, 202 to 487 million years old.
The section below crosses 12 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 5×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Argillite, quartzite, phyllite. Age: Ordovician to Triassic, 202 to 487 million years old. About 7 mi of the line.
Granodiorite. Age: Permian to Tertiary, 2.6 to 260 million years old. About 6 mi of the line.
Metasedimentary, metavolcanic. Age: Ordovician to Jurassic, 145 to 487 million years old. About 2 mi of the line.
Peridotite, serpentinite. Age: Neoproterozoic to Early Jurassic, 175 to 1,000 million years old. About 2 mi of the line.
Slate, graywacke. Age: Permian, 252 to 299 million years old. About 1 mi of the line.
Andesite. Age: Devonian to Permian, 252 to 422 million years old. About 0.9 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 6 smaller units along the line are not listed.
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 Twain reaches San Francisco Bay and the Pacific Ocean by way of the East Branch North Fork Feather, the North Fork Feather, the Feather and the Sacramento: about 220 river miles.
75 miles of the trip are on the Sacramento.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Twain; river miles along the main stem, rounded.
Twain has a warm-summer Mediterranean climate. Summer afternoons average 89° in July; winter nights fall to about 25° in January. A normal year brings about 28 inches of snow; with the rain, that comes to about 42 inches of water.
On November 14 Twain normally reaches 53° and falls to 29°. The record high is 80°, in 1921; the record low 5°, in 1985. The most rain on this date, 3.35 in, fell in 2025.
Normals 1991–2020 and records since 1895 at Quincy, 9 miles southeast, 520 ft higher. NOAA, through RCC-ACIS.
