The Helena 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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4.71 inches of rain fell in two days, the most for this week of the year since records began in 1893. The next wettest, 3.51 inches, fell in 1940.
Helena’s center stands about 530 ft above sea level. Within three miles the land runs from 430 ft to 590 ft, a difference of 160 ft: gently rolling country. The highest named summit within 15 miles is Little Mountain, about 830 ft, 15 miles east-southeast.
Under the town center the state geologic map shows Newberry granite, from the Silurian, 422 to 445 million years old.
The section below crosses 4 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 64×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Granite. Age: Silurian, 422 to 445 million years old. About 10 mi of the line.
Metavolcanic, gneiss. Age: Neoproterozoic to Ordovician, 445 to 1,000 million years old. About 9 mi of the line.
Mylonite. Age: Carboniferous to Permian, 252 to 359 million years old. About 3 mi of the line.
Biotite-gneiss. Age: Neoproterozoic to Paleozoic, 252 to 1,000 million years old. About 3 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 Helena reaches the Atlantic Ocean by way of the Bush, the Saluda, the Congaree, the Santee and the South Santee: about 240 river miles.
Lakes on the way: Lib Pines Lake and Lake Murray.
70 miles of the trip are on the Santee.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Helena; river miles along the main stem, rounded.
Helena has a humid subtropical climate. Summer afternoons average 92° in July; winter nights fall to about 31° in January. About 47 inches of rain fall in a normal year.
On November 14 Helena normally reaches 64° and falls to 40°. The record high is 84°, in 1931; the record low 23°, in 2013. The most rain on this date, 1.67 in, fell in 1997.
Normals 1991–2020 and records since 1893 at Newberry, here. NOAA, through RCC-ACIS.
