The Altadena 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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95° on Nov. 14, 2016, the hottest reading for this week of the year since 1893. Next is 95° in 2021.
Altadena’s center stands about 1,410 ft above sea level. Within three miles the land runs from 840 ft to 4,510 ft, a difference of 3,670 ft: mountain country. The highest named summit within 15 miles is Twin Peaks, about 7,750 ft, 15 miles northeast.
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 7 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.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 6 mi of the line.
Quartz-monzonite. Age: Late Triassic, 218 to 218 million years old. About 5 mi of the line.
Granodiorite. Age: Permian to Tertiary, 2.6 to 260 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.
Orthogneiss. Age: Paleoproterozoic to Miocene, 5.3 to 2,500 million years old. About 3 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 2 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 1 smaller unit 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 Altadena reaches the Pacific Ocean by way of Alhambra Wash, the Rio Hondo and the Los Angeles: about 30 river miles.
Often dry. About 50% 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 Altadena; river miles along the main stem, rounded.
Altadena has a hot-summer Mediterranean climate. Summer afternoons average 91° in August; winter nights fall to about 45° in December. About 20 inches of rain fall in a normal year.
On November 14 Altadena normally reaches 75° and falls to 50°. The record high is 95°, in 2016; the record low 29°, in 1916. The most rain on this date, 1.30 in, fell in 1928.
Normals 1991–2020 and records since 1893 at Pasadena, 3 miles south, 550 ft lower. NOAA, through RCC-ACIS.
