The Alto 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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6.85 inches of rain fell in two days, the most for this week of the year since records began in 1902. The next wettest, 6.57 inches, fell in 1942.
Alto’s center stands about 30 ft above sea level. Within three miles the land runs from 0 ft to 930 ft, a difference of 930 ft: steep hill country. Open water lies within three miles. The highest named summit within 15 miles is Mount Tamalpais, about 2,570 ft, 4 miles west-northwest.
Under the town center the state geologic map shows Franciscan melange, from the Jurassic to Cretaceous, 66 to 202 million years old.
The section below crosses 5 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 8×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Melange. Age: Jurassic to Cretaceous, 66 to 202 million years old. About 10 mi of the line.
Sandstone, mudstone. Age: Jurassic to Cretaceous, 66 to 202 million years old. About 3 mi of the line.
Greenstone. Age: Jurassic to Cretaceous, 66 to 202 million years old. About 2 mi of the line.
Serpentinite. Age: Middle Jurassic to Late Jurassic, 145 to 175 million years old. About 1 mi of the line.
Mica-schist. Age: Early Cretaceous, 110 to 143 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.
Rain that falls on Alto runs straight into San Francisco Bay and the Pacific Ocean.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Alto; river miles along the main stem, rounded.
Alto has a warm-summer Mediterranean climate. Summer afternoons average 83° in July; winter nights fall to about 42° in January. About 44 inches of rain fall in a normal year.
On November 14 Alto normally reaches 64° and falls to 46°. The record high is 81°, in 2010; the record low 25°, in 1916. The most rain on this date, 3.10 in, fell in 1952.
Normals 1991–2020 and records since 1902 at Kentfield, 4 miles north-northwest, 120 ft higher. NOAA, through RCC-ACIS.
