The Lower Gwynedd 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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77° on Nov. 12, 2015, the hottest reading for this week of the year since 1948. Next is 76° in 2020.
Lower Gwynedd’s center stands about 310 ft above sea level. Within three miles the land runs from 200 ft to 480 ft, a difference of 280 ft: rolling country. The highest named summit within 15 miles is Mount Misery, about 700 ft, 15 miles west-southwest.
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 29×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Argillite. Age: Triassic, 202 to 252 million years old. About 8 mi of the line.
Arkose. Age: Triassic, 202 to 252 million years old. About 5 mi of the line.
Mica-schist. Age: Paleozoic, 252 to 542 million years old. About 5 mi of the line.
Mudstone. Age: Triassic, 202 to 252 million years old. About 4 mi of the line.
Limestone. Age: Cambrian to Ordovician, 445 to 542 million years old. About 0.8 mi of the line.
Quartzite, schist. Age: Cambrian, 487 to 542 million years old. About 0.8 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 Lower Gwynedd reaches Delaware Bay and the Atlantic Ocean by way of Trewellyn Creek, Wissahickon Creek, the Schuylkill and the Delaware: about 90 river miles.
60 miles of the trip are on the Delaware.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Lower Gwynedd; river miles along the main stem, rounded.
Lower Gwynedd has a humid subtropical climate. Summer afternoons average 89° in July; winter nights fall to about 23° in January. A normal year brings about 25 inches of snow; with the rain, that comes to about 51 inches of water.
On November 14 Lower Gwynedd normally reaches 57° and falls to 35°. The record high is 74°, in 1955; the record low 19°, in 1986. The most rain on this date, 0.70 in, fell in 2020.
Normals 1991–2020 and records since 1948 at Norristown, 8 miles southwest, 250 ft lower. NOAA, through RCC-ACIS.
