The Dewey 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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−20° on Nov. 13, 2014, the coldest reading for this week of the year since 1943. Next is −12° in 2000.
Dewey’s center stands about 5,530 ft above sea level. Within three miles the land runs from 5,500 ft to 8,260 ft, a difference of 2,760 ft: mountain country. The highest named summit within 15 miles is Granite Mountain, about 10,590 ft, 15 miles south-southwest.
Under the town center the state geologic map shows Snowcrest Range and Madison Groups; or Snowcrest Range and Tendoy Groups; or Surret Canyon through McGowan Creek Formation: Surrett Canyon, South Creek, Scott Peak, Middle Canyon, and McGowan Creek Formations (limestone), from the Early Mississippian to Early Pennsylvanian, 315 to 359 million years old.
The section below crosses 11 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.
Shale, limestone, sandstone. Age: Middle Cambrian to Cambrian, 487 to 521 million years old. About 5 mi of the line.
Granitic. Age: Cretaceous, 66 to 145 million years old. About 5 mi of the line.
Limestone. Age: Early Mississippian to Early Pennsylvanian, 315 to 359 million years old. About 3 mi of the line.
Sedimentary. Age: Tertiary, 2.6 to 66 million years old. About 3 mi of the line.
Sandstone, shale, siltstone. Age: Early Cretaceous to Late Cretaceous, 90 to 113 million years old. About 3 mi of the line.
Granitic. Age: Tertiary, 2.6 to 66 million years old. About 2 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 5 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 Dewey reaches the Gulf of America by way of the Big Hole, the Jefferson, the Missouri and the Mississippi: about 3,610 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 Dewey; river miles along the main stem, rounded.
Dewey has a warm-summer humid continental climate. Summer afternoons average 79° in July; winter nights fall to about 7° in December. About 13 inches of rain fall in a normal year.
Normals 1991–2020 and records since 1943 at Wise River 3 WNW, 7 miles west-northwest, 200 ft higher. NOAA, through RCC-ACIS.
