The Keystone 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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Chance of showers
−19° on Nov. 13, 2014, the coldest reading for this week of the year since 1909. Next is −12° in 2019.
Keystone’s center stands about 4,500 ft above sea level. Within three miles the land runs from 4,200 ft to 5,500 ft, a difference of 1,300 ft: steep hill country. The highest named summit within 15 miles is Black Elk Peak, about 7,150 ft, 6 miles west-southwest.
Under the town center the state geologic map shows Metagraywacke (mica-schist, quartzite), from the Paleoproterozoic, 1.6 to 2.5 billion years old.
The section below crosses 12 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 5×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Mica-schist, quartzite. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 8 mi of the line.
Granite. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 7 mi of the line.
Claystone, siltstone, sandstone. Age: Eocene to Oligocene, 23 to 56 million years old. About 2 mi of the line.
Sandstone, siltstone, shale. Age: Late Pennsylvanian to Permian, 273 to 307 million years old. About 2 mi of the line.
Claystone, conglomerate, sandstone. Age: Early Cretaceous, 100 to 145 million years old. About 1 mi of the line.
Limestone, dolostone. Age: Late Devonian to Early Mississippian, 347 to 384 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. 6 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 Keystone reaches the Gulf of America by way of Battle Creek, the Cheyenne, the Missouri and the Mississippi: about 2,540 river miles.
Lakes on the way: Lake Oahe, Lake Sharpe, Lake Francis Case and Lewis and Clark Lake.
1,170 miles of the trip are on the Mississippi.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Keystone; river miles along the main stem, rounded.
Keystone has a dry-winter warm-summer continental climate. Summer afternoons average 83° in July; winter nights fall to about 10° in January. A normal year brings about 67 inches of snow; with the rain, that comes to about 22 inches of water.
On November 14 Keystone normally reaches 49° and falls to 18°. The record high is 73°, in 2025; the record low −19°, in 2014. The most rain on this date, 0.25 in, fell in 1956.
Normals 1991–2020 and records since 1909 at Hill City, 7 miles west-northwest, 450 ft higher. NOAA, through RCC-ACIS.
