The Union Mill 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.
Union Mill’s center stands about 330 ft above sea level. Within three miles the land runs from 180 ft to 470 ft, a difference of 290 ft: rolling country. The highest named summit within 15 miles is Gantt Hill, about 510 ft, 12 miles northeast.
Under the town center the state geologic map shows Popes Head Formation - Old Mill Branch Metasiltstone Member, from the Cambrian to Ordovician, 445 to 542 million years old.
The section below crosses 12 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 51×. 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, siltstone. Age: Triassic, 202 to 252 million years old. About 5 mi of the line.
Basalt. Age: Early Jurassic, 175 to 202 million years old. About 4 mi of the line.
Granite. Age: Cambrian to Ordovician, 445 to 542 million years old. About 4 mi of the line.
Melange. Age: Cambrian, 487 to 542 million years old. About 3 mi of the line.
Metasiltstone. Age: Cambrian to Ordovician, 445 to 542 million years old. About 3 mi of the line.
Schist. Age: Neoproterozoic to Cambrian, 487 to 1,000 million years old. About 1 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 6 smaller units 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 Union Mill reaches Chesapeake Bay and the Atlantic Ocean by way of Castle Creek, Popes Head Creek, Bull Run, the Occoquan and the Potomac: about 110 river miles.
85 miles of the trip are on the Potomac.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Union Mill; river miles along the main stem, rounded.
Union Mill 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 Union Mill 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, 10 miles north-northwest). NOAA, through RCC-ACIS.
