The San Marcos 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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96° on Nov. 12, 2021, the hottest reading for this week of the year since 1893. Next is 94° in 2019.
San Marcos’s center stands about 590 ft above sea level. Within three miles the land runs from 470 ft to 1,420 ft, a difference of 950 ft: steep hill country. The highest named summit within 15 miles is Woodson Mountain, about 2,830 ft, 15 miles southeast.
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 5 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 9×. 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 12 mi of the line.
Mudstone, sandstone. Age: Paleocene to Oligocene, 23 to 66 million years old. About 4 mi of the line.
Argillite. Age: Paleozoic to Late Jurassic, 145 to 542 million years old. About 3 mi of the line.
Siltstone-mudstone. Age: Early Cretaceous to Late Cretaceous, 66 to 145 million years old. About 1 mi of the line.
Coarse-detrital. Age: Pleistocene to Holocene, less than 2.6 million years old. About 0.4 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 San Marcos reaches the Pacific Ocean by way of San Marcos Creek: about 11 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 San Marcos; river miles along the main stem, rounded.
San Marcos has a hot semi-arid climate. Summer afternoons average 89° in August; winter nights fall to about 44° in December. About 14 inches of rain fall in a normal year.
On November 14 San Marcos normally reaches 75° and falls to 50°. The record high is 93°, in 1949; the record low 24°, in 1938. The most rain on this date, 1.47 in, fell in 1950.
Normals 1991–2020 and records since 1893 from the Escondido Area threaded record (Escondido No. 2, 5 miles east-southeast). NOAA, through RCC-ACIS.
