The Casa de Oro-Mount Helix 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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3.00 inches of rain fell in two days, the most for this week of the year since records began in 1899. The next wettest, 2.34 inches, fell in 1944.
Casa de Oro-Mount Helix’s center stands about 830 ft above sea level. Within three miles the land runs from 330 ft to 970 ft, a difference of 640 ft: hilly country. The highest named summit within 15 miles is Gaskill Peak, about 3,770 ft, 15 miles east.
Under the town center the state geologic map shows Mesozoic granitic rocks, unit 2 (Peninsular Ranges) (tonalite), from the Middle Jurassic to Late Cretaceous, 66 to 175 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 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.
Tonalite. Age: Middle Jurassic to Late Cretaceous, 66 to 175 million years old. About 14 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 3 mi of the line.
Conglomerate, sandstone. Age: Middle Eocene to Late Eocene, 34 to 48 million years old. About 3 mi of the line.
Felsic-volcanic, mafic-volcanic. Age: Late Jurassic to Early Cretaceous, 100 to 164 million years old. About 2 mi of the line.
Mudstone, sandstone. Age: Paleocene to Oligocene, 23 to 66 million years old. About 1 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 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.
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 Casa de Oro-Mount Helix reaches the Pacific Ocean by way of the Sweetwater: about 16 river miles.
Often dry. About 60% 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 Casa de Oro-Mount Helix; river miles along the main stem, rounded.
Casa de Oro-Mount Helix has a hot semi-arid climate. Summer afternoons average 89° in August; winter nights fall to about 42° in December. About 12 inches of rain fall in a normal year.
On November 14 Casa de Oro-Mount Helix normally reaches 76° and falls to 48°. The record high is 98°, in 1906; the record low 25°, in 1916. The most rain on this date, 1.07 in, fell in 1950.
Normals 1991–2020 and records since 1899 from the El Cajon Area threaded record (El Cajon, 3 miles northeast, 340 ft lower). NOAA, through RCC-ACIS.
