The Gilliam 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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Gilliam’s center stands about 200 ft above sea level. Within three miles the land runs from 170 ft to 260 ft, a difference of 90 ft: gently rolling country. The highest named summit within 15 miles is Delaney Mount, about 480 ft, 9 miles northeast.
Under the town center the state geologic map shows Natural Levees (silt, clay), from the Holocene, less than 12,000 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.
Silt, clay. Age: Holocene, less than 12,000 years old. About 8 mi of the line.
Clay. Age: Pleistocene, less than 2.6 million years old. About 5 mi of the line.
Clay, marl. Age: Eocene, 34 to 56 million years old. About 4 mi of the line.
Clay, silt, sand. Age: Pleistocene, less than 2.6 million years old. About 2 mi of the line.
Sand, clay. Age: Eocene, 34 to 56 million years old. About 2 mi of the line.
Clay. Age: Holocene, less than 12,000 years old. About 0.8 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 Gilliam reaches the Gulf of America by way of Red Bayou, Black Bayou, Twelvemile Bayou, Cross Bayou, the Red River, the Atchafalaya and the Lower Atchafalaya: about 440 river miles.
260 miles of the trip are on the Red River.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Gilliam; river miles along the main stem, rounded.
