The Lake 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.
Lake San Marcos’s center stands about 500 ft above sea level. Within three miles the land runs from 200 ft to 1,310 ft, a difference of 1,110 ft: steep hill country. The highest named summit within 15 miles is Burnt Mountain, about 2,090 ft, 10 miles northeast.
Under the town center the state geologic map shows Eocene marine rocks (mudstone, sandstone), from the Paleocene to Oligocene, 23 to 66 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 11×. 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 7 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.
Felsic-volcanic, mafic-volcanic. Age: Late Jurassic to Early Cretaceous, 100 to 164 million years old. About 2 mi of the line.
Coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 0.9 mi of the line.
Diorite, gabbro. Age: Triassic to Cretaceous, 66 to 252 million years old. About 0.6 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 Lake San Marcos reaches the Pacific Ocean by way of San Marcos Creek: about 9 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Lake San Marcos; river miles along the main stem, rounded.
Lake 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 Lake 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, 7 miles east, 100 ft higher). NOAA, through RCC-ACIS.
