The St. Helena 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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22° on Nov. 15, 2000, the coldest reading for this week of the year since 1906. Next is 24° in 1985.
St. Helena’s center stands about 220 ft above sea level. Within three miles the land runs from 180 ft to 1,450 ft, a difference of 1,270 ft: steep hill country. The highest named summit within 15 miles is Mount Saint Helena, about 4,310 ft, 14 miles northwest.
Under the town center the state geologic map shows Quaternary alluvium and marine deposits (coarse-detrital), from the Pleistocene to Holocene, less than 2.6 million years old.
The section below crosses 8 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 7×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Rhyolite, dacite. Age: Late Miocene to Late Pliocene, 3 to 7 million years old. About 7 mi of the line.
Sandstone, siltstone-mudstone. Age: Late Cretaceous, 66 to 100 million years old. About 6 mi of the line.
Dacite. Age: Late Miocene to Late Pliocene, 3 to 7 million years old. About 4 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 3 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 3 mi of the line.
Melange. Age: Jurassic to Cretaceous, 66 to 202 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. 2 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 St. Helena reaches San Francisco Bay and the Pacific Ocean by way of the Napa: about 40 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of St. Helena; river miles along the main stem, rounded.
St. Helena has a hot-summer Mediterranean climate. Summer afternoons average 90° in July; winter nights fall to about 38° in December. About 39 inches of rain fall in a normal year.
On November 14 St. Helena normally reaches 66° and falls to 42°. The record high is 88°, in 2008; the record low 26°, in 1985. The most rain on this date, 2.48 in, fell in 1953.
Normals 1991–2020 and records since 1906 at Calistoga, 7 miles northwest, 130 ft higher. NOAA, through RCC-ACIS.
