The Rosemont 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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82° on Nov. 16, 1931, the hottest reading for this week of the year since 1900. Next is 81° in 1909.
Rosemont’s center stands about 460 ft above sea level. Within three miles the land runs from 210 ft to 1,100 ft, a difference of 890 ft: hilly country. The highest named summit within 15 miles is Lambs Knoll, about 1,760 ft, 8 miles north.
Under the town center the state geologic map shows Granodiorite and Biotite Granite Gneiss, from the Precambrian.
The section below crosses 9 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 13×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Gneiss. Age: Precambrian. About 6 mi of the line.
Metagranite. Age: Mesoproterozoic, 1 to 1.6 billion years old. About 4 mi of the line.
Quartzite. Age: Proterozoic to Cambrian, 487 to 2,500 million years old. About 3 mi of the line.
Leucocratic-granitic. Age: Mesoproterozoic, 1.1 to 1.1 billion years old. About 3 mi of the line.
Metabasalt. Age: Proterozoic to Mesoproterozoic, 1 to 2.5 billion years old. About 2 mi of the line.
Metadiorite. Age: Mesoproterozoic, 1 to 1.6 billion years old. About 2 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 3 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 Rosemont reaches Chesapeake Bay and the Atlantic Ocean by way of Little Catoctin Creek and the Potomac: about 170 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Rosemont; river miles along the main stem, rounded.
Rosemont has a humid subtropical climate. Summer afternoons average 86° in July; winter nights fall to about 24° in January. About 44 inches of rain fall in a normal year.
On November 14 Rosemont normally reaches 56° and falls to 35°. The record high is 77°, in 1955; the record low 15°, in 1986. The most rain on this date, 1.74 in, fell in 1970.
Normals 1991–2020 and records since 1900 at Purcellville, 14 miles south-southwest. NOAA, through RCC-ACIS.
