The Rancho Santa Margarita 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. 17, 1958, the coldest reading for this week of the year since 1897. Next is 25° in 1964.
Rancho Santa Margarita’s center stands about 950 ft above sea level. Within three miles the land runs from 650 ft to 1,550 ft, a difference of 900 ft: hilly country. The highest named summit within 15 miles is Santiago Peak, about 5,660 ft, 6 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 9 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.
Argillite. Age: Paleozoic to Late Jurassic, 145 to 542 million years old. About 7 mi of the line.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 6 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.
Felsic-volcanic, mafic-volcanic. Age: Late Jurassic to Early Cretaceous, 100 to 164 million years old. About 1 mi of the line.
Mudstone, sandstone. Age: Paleocene to Oligocene, 23 to 66 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. 3 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 Rancho Santa Margarita reaches the Pacific Ocean by way of the Arroyo Trabuco, Trabuco Creek and San Juan Creek: about 13 river miles.
Often dry. About 100% 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 Rancho Santa Margarita; river miles along the main stem, rounded.
Rancho Santa Margarita has a hot semi-arid climate. Summer afternoons average 98° in August; winter nights fall to about 40° in December. About 12 inches of rain fall in a normal year.
On November 14 Rancho Santa Margarita normally reaches 74° and falls to 46°. The record high is 94°, in 1906; the record low 25°, in 1964. The most rain on this date, 0.97 in, fell in 1944.
Normals 1991–2020 and records since 1897 at Elsinore, 15 miles east-northeast, 320 ft higher. NOAA, through RCC-ACIS.
