The Potomac 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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81° on Nov. 15, 1993, the hottest reading for this week of the year since 1960. Next is 80° in 1985.
Potomac’s center stands about 360 ft above sea level. Within three miles the land runs from 110 ft to 420 ft, a difference of 310 ft: rolling country. The highest named summit within 15 miles is Hunting Hill, about 520 ft, 5 miles north.
Under the town center the state geologic map shows Wissahickon Formation (Undivided) (muscovite-schist), from the Proterozoic, 542 to 2,500 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 48×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Muscovite-schist. Age: Proterozoic, 542 to 2,500 million years old. About 15 mi of the line.
Schist. Age: Neoproterozoic to Cambrian, 487 to 1,000 million years old. About 3 mi of the line.
Metagraywacke. Age: Neoproterozoic to Cambrian, 487 to 1,000 million years old. About 2 mi of the line.
Tonalite. Age: Cambrian to Ordovician, 445 to 542 million years old. About 2 mi of the line.
Schist, metagraywacke. Age: Neoproterozoic to Cambrian, 487 to 1,000 million years old. About 0.8 mi of the line.
Melange. Age: Cambrian, 487 to 542 million years old. About 0.5 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 Potomac reaches Chesapeake Bay and the Atlantic Ocean by way of Cabin John Creek and the Potomac: about 130 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Potomac; river miles along the main stem, rounded.
Potomac has a humid subtropical climate. Summer afternoons average 88° in July; winter nights fall to about 25° in January. A normal year brings about 21 inches of snow; with the rain, that comes to about 43 inches of water.
On November 14 Potomac normally reaches 57° and falls to 36°. The record high is 80°, in 1985; the record low 13°, in 1986. The most rain on this date, 1.90 in, fell in 1972.
Normals 1991–2020 and records since 1960 from the Sterling-Dulles Airport Area threaded record (Washington Dulles International Airport, 14 miles west-southwest). NOAA, through RCC-ACIS.
