The Grand Mound 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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5.38 inches of rain fell in two days, the most for this week of the year since records began in 1948. The next wettest, 5.30 inches, fell in 2015.
Grand Mound’s center stands about 160 ft above sea level. Within three miles the land runs from 130 ft to 560 ft, a difference of 430 ft: hilly country. The highest named summit within 15 miles is Capitol Peak, about 2,660 ft, 14 miles north-northwest.
Under the town center the state geologic map shows Younger glacial drift (fine-detrital, coarse-detrital), from the Pleistocene, less than 2.6 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 14×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Fine-detrital, coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 9 mi of the line.
Sandstone, siltstone. Age: Middle Eocene to Early Miocene, 16 to 48 million years old. About 6 mi of the line.
Basalt. Age: Early Paleocene to Middle Eocene, 40 to 64 million years old. About 5 mi of the line.
Silt, sand. Age: Holocene, less than 12,000 years old. About 2 mi of the line.
Fine-detrital, coarse-detrital. Age: Pleistocene, less than 2.6 million years old. About 2 mi of the line.
Siltstone, sandstone. Age: Middle Eocene to Late Eocene, 34 to 48 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.
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 Grand Mound reaches the Pacific Ocean by way of Prairie Creek and the Chehalis: about 60 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Grand Mound; river miles along the main stem, rounded.
Grand Mound has a warm-summer Mediterranean climate. Summer afternoons average 78° in July; winter nights fall to about 32° in February. About 51 inches of rain fall in a normal year, with about 4 inches of snow.
On November 14 Grand Mound normally reaches 51° and falls to 36°. The record high is 65°, in 1995; the record low 3°, in 1955. The most rain on this date, 3.64 in, fell in 2001.
Normals 1991–2020 and records since 1948 from the Olympia Area threaded record (Olympia Airport, 14 miles north-northeast). NOAA, through RCC-ACIS.
