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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10° on Nov. 17, 2014, the coldest reading for this week of the year since 1906. Next is 11° in 2019.
Helena’s center stands about 1,410 ft above sea level. Within three miles the land runs from 1,340 ft to 1,430 ft, a difference of 90 ft: gently rolling country.
Under the town center the state geologic map shows Flowerpot Shale (shale, siltstone), from the Permian, 252 to 299 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 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.
Coarse-detrital, fine-detrital. Age: Pleistocene, less than 2.6 million years old. About 11 mi of the line.
Shale, siltstone. Age: Permian, 252 to 299 million years old. About 6 mi of the line.
Sandstone, shale. Age: Permian, 252 to 299 million years old. About 4 mi of the line.
Shale, sandstone, siltstone. Age: Permian, 252 to 299 million years old. About 2 mi of the line.
Shale, sandstone, siltstone. Age: Permian, 252 to 299 million years old. About 2 mi of the line.
Fine-detrital. Age: Holocene, less than 12,000 years old. About 0.3 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 Gulf of America by way of Turkey Creek, the Cimarron, the Arkansas and the Mississippi: about 1,340 river miles.
Lakes on the way: Murray Lake and Ozark Lake.
600 miles of the trip are on the Mississippi.
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 95° in July; winter nights fall to about 23° in January. A normal year brings about 13 inches of snow; with the rain, that comes to about 32 inches of water.
On November 14 Helena normally reaches 60° and falls to 36°. The record high is 82°, in 1999; the record low 12°, in 2018. The most rain on this date, 0.66 in, fell in 1985.
Normals 1991–2020 and records since 1906 at Helena 1 SSE, here. NOAA, through RCC-ACIS.
