The Forsan 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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Forsan’s center stands about 2,790 ft above sea level. Within three miles the land runs from 2,630 ft to 2,790 ft, a difference of 160 ft: gently rolling country.
Under the town center the state geologic map shows Edwards Limestone, from the Early Cretaceous, 100 to 113 million years old.
The section below crosses 6 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 33×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Coarse-detrital, fine-detrital. Age: Quaternary, less than 2.6 million years old. About 11 mi of the line.
Limestone. Age: Early Cretaceous, 100 to 113 million years old. About 7 mi of the line.
Shale. Age: Middle Triassic to Late Triassic, 208 to 247 million years old. About 4 mi of the line.
Sandstone, claystone. Age: Early Cretaceous, 100 to 145 million years old. About 1 mi of the line.
Sand. Age: Pleistocene, less than 2.6 million years old. About 0.9 mi of the line.
Sand. Age: Pleistocene to Holocene, less than 2.6 million years old. About 0.4 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 Forsan reaches the Gulf of America by way of the North Concho, the Concho and the Colorado: about 780 river miles.
Lakes on the way: Buchanan Lake, Lake Marble Falls and Lake Lyndon B Johnson.
320 miles of the trip are on the Colorado.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Forsan; river miles along the main stem, rounded.
