The Shamrock 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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68° on Nov. 17, 1953, the hottest reading for this week of the year since 1892. Next is 68° in 1999.
Shamrock’s center stands about 1,240 ft above sea level. Within three miles the land runs from 1,210 ft to 1,330 ft, a difference of 120 ft: gently rolling country.
Under the town center the state geologic map shows Slate and graywacke (slate, graywacke), from the Paleoproterozoic, 1.6 to 2 billion years old.
The section below crosses 7 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.
Slate, graywacke. Age: Paleoproterozoic, 1.6 to 2 billion years old. About 17 mi of the line.
Quartzite. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 2 mi of the line.
Volcanic, sedimentary. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 2 mi of the line.
Siltstone, graywacke, slate. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 1 mi of the line.
Graywacke, slate. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 1 mi of the line.
Graywacke, slate. Age: Paleoproterozoic, 1.6 to 2.5 billion years old. About 0.6 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 1 smaller unit 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 Shamrock reaches the Gulf of America by way of Minnewawa Creek, the Sandy and the Mississippi: about 2,100 river miles.
Lakes on the way: Lake Pepin and Aitkin Lake.
1,750 miles of the trip are on the Mississippi.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Shamrock; river miles along the main stem, rounded.
Shamrock has a cold desert climate. Summer afternoons average 78° in July; winter nights fall to about −2° in January. Barely 0.0 inches of rain fall in a normal year.
On November 14 Shamrock normally reaches 38° and falls to 23°. The record high is 64°, in 2005; the record low −7°, in 1919. The most rain on this date, 1.24 in, fell in 1909.
Normals 1991–2020 and records since 1892 at Sandy Lake Dam Libby, 5 miles north-northwest. NOAA, through RCC-ACIS.
