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.
Chance of showers
Showers likely
Mostly cloudy; cooler
Mostly sunny
Mostly cloudy
Chance of showers
Partly cloudy
5.75 inches of rain fell in two days, the most for this week of the year since records began in 1893. The next wettest, 3.12 inches, fell in 2001.
San Marcos’s center stands about 620 ft above sea level. Within three miles the land runs from 560 ft to 830 ft, a difference of 270 ft: rolling country. The highest named summit within 15 miles is Tower Hill, about 1,320 ft, 13 miles west-northwest.
Under the town center the state geologic map shows Terrace deposits (sand), from the Pleistocene to Holocene, less than 2.6 million years old.
The section below crosses 7 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 25×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Limestone. Age: Early Cretaceous, 100 to 113 million years old. About 9 mi of the line.
Sand. Age: Pleistocene to Holocene, less than 2.6 million years old. About 7 mi of the line.
Clay, siltstone-mudstone. Age: Late Cretaceous, 66 to 100 million years old. About 4 mi of the line.
Fine-detrital, coarse-detrital. Age: Holocene, less than 12,000 years old. About 2 mi of the line.
Limestone, marlstone. Age: Early Cretaceous, 100 to 145 million years old. About 2 mi of the line.
Siltstone-mudstone. Age: Late Cretaceous, 66 to 100 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. 1 smaller unit 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 San Marcos reaches the Gulf of America by way of Willow Springs Creek, the San Marcos, the Guadalupe and the South Guadalupe: about 250 river miles.
180 miles of the trip are on the Guadalupe.
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 humid subtropical climate. Summer afternoons average 97° in August; winter nights fall to about 39° in January. About 37 inches of rain fall in a normal year.
On November 14 San Marcos normally reaches 72° and falls to 49°. The record high is 89°, in 1951; the record low 24°, in 2018. The most rain on this date, 2.03 in, fell in 1959.
Normals 1991–2020 and records since 1893 at San Marcos, here. NOAA, through RCC-ACIS.
