The Alum Rock 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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84° on Nov. 17, 1932, the hottest reading for this week of the year since 1893. Next is 83° in 2008.
Alum Rock’s center stands about 170 ft above sea level. Within three miles the land runs from 90 ft to 1,780 ft, a difference of 1,690 ft: steep hill country. The highest named summit within 15 miles is Copernicus Peak, about 4,330 ft, 11 miles east.
Under the town center the state geologic map shows Older Quaternary alluvium and marine deposits (coarse-detrital), from the Pleistocene, less than 2.6 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 6×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 9 mi of the line.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 5 mi of the line.
Sandstone, mudstone. Age: Jurassic to Cretaceous, 66 to 202 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.
Melange. Age: Jurassic to Cretaceous, 66 to 202 million years old. About 1 mi of the line.
Siltstone-mudstone. Age: Early Cretaceous to Late Cretaceous, 66 to 145 million years old. About 1 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 Alum Rock reaches San Francisco Bay and the Pacific Ocean by way of Silver Creek and Coyote Creek: about 14 river miles.
Often dry. About 30% of the way is mapped as streams that run only after rain or snowmelt; the water makes the whole trip in floods.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Alum Rock; river miles along the main stem, rounded.
Alum Rock has a warm-summer Mediterranean climate. Summer afternoons average 82° in August; winter nights fall to about 42° in January. About 14 inches of rain fall in a normal year.
On November 14 Alum Rock normally reaches 66° and falls to 46°. The record high is 83°, in 2008; the record low 27°, in 1916. The most rain on this date, 1.23 in, fell in 1972.
Normals 1991–2020 and records since 1893 from the San Jose Area threaded record (San Jose, 6 miles west, 120 ft lower). NOAA, through RCC-ACIS.
