The East 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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75° on Nov. 12, 1999, the hottest reading for this week of the year since 1880. Next is 72° in 1990.
East Helena’s center stands about 3,920 ft above sea level. Within three miles the land runs from 3,770 ft to 4,330 ft, a difference of 560 ft: hilly country. The highest named summit within 15 miles is High Peak, about 8,520 ft, 13 miles south-southeast.
The section below crosses 9 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 4×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Age: Pleistocene to Holocene, less than 1.8 million years old. About 12 mi of the line.
Granitic. Age: Cretaceous, 66 to 145 million years old. About 5 mi of the line.
Andesite, latite. Age: Late Cretaceous, 72 to 84 million years old. About 2 mi of the line.
Age: Cretaceous, 66 to 145 million years old. About 1 mi of the line.
Siltstone, sedimentary-breccia, conglomerate. Age: Early Eocene to Early Miocene, 16 to 56 million years old. About 1 mi of the line.
Quartzite, argillite, metasiltstone. Age: Mesoproterozoic, 1 to 1.6 billion 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. 3 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 East Helena reaches the Gulf of America by way of Prickly Pear Creek, the Missouri and the Mississippi: about 3,410 river miles.
Lakes on the way: Lake Oahe, Lake Sakakawea, Lake Fort Peck and Lake Sharpe.
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 East Helena; river miles along the main stem, rounded.
East Helena has a cold semi-arid climate. Summer afternoons average 86° in July; winter nights fall to about 13° in January. A normal year brings about 37 inches of snow; with the rain, that comes to about 11 inches of water.
On November 14 East Helena normally reaches 43° and falls to 23°. The record high is 66°, in 2021; the record low −20°, in 1955. The most rain on this date, 0.48 in, fell in 1903.
Normals 1991–2020 and records since 1880 from the Helena Area threaded record (Helena Airport Asos, 4 miles west-northwest). NOAA, through RCC-ACIS.
