The Rockport Colony 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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Rockport Colony’s center stands about 4,410 ft above sea level. Within three miles the land runs from 4,230 ft to 4,640 ft, a difference of 410 ft: hilly country. The highest named summit within 15 miles is Choteau Mountain, about 8,340 ft, 14 miles southwest.
Under the town center the state geologic map shows Two Medicine Formation (sandstone, siltstone, mudstone), from the Late Cretaceous, 66 to 84 million years old.
The section below crosses 4 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 8×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Sandstone, siltstone, mudstone. Age: Late Cretaceous, 66 to 84 million years old. About 14 mi of the line.
Age: Late Miocene to Holocene, less than 7.2 million years old. About 9 mi of the line.
Siltstone, sandstone, shale. Age: Middle Jurassic to Late Cretaceous, 94 to 172 million years old. About 0.8 mi of the line.
Limestone, dolostone, black-shale. Age: Early Mississippian to Middle Mississippian, 331 to 359 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 Rockport Colony reaches the Gulf of America by way of the South Fork Dry Fork Marias, the Dry Fork Marias, the Marias, the Missouri and the Mississippi: about 3,400 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 Rockport Colony; river miles along the main stem, rounded.
