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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Shamrock’s center stands about 2,340 ft above sea level. Within three miles the land runs from 2,160 ft to 2,390 ft, a difference of 230 ft: rolling country.
Under the town center the state geologic map shows Blaine Formation (shale), from the Permian, 252 to 299 million years old.
The section below crosses 5 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 31×. 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. Age: Permian, 252 to 299 million years old. About 18 mi of the line.
Coarse-detrital, fine-detrital. Age: Quaternary, less than 2.6 million years old. About 2 mi of the line.
Sand. Age: Holocene, less than 12,000 years old. About 2 mi of the line.
Sand. Age: Late Miocene to Pliocene, 2.6 to 12 million years old. About 2 mi of the line.
Sandstone-mudstone. Age: Permian, 252 to 299 million 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.
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 the North Fork Red, the Red River, the Atchafalaya and the Lower Atchafalaya: about 1,300 river miles.
Lake on the way: Lake Altus.
960 miles of the trip are on the Red River.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Shamrock; river miles along the main stem, rounded.
