The Grinnell 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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78° on Nov. 14, 1999, the hottest reading for this week of the year since 1893. Next is 76° in 1948.
Grinnell’s center stands about 1,020 ft above sea level. Within three miles the land runs from 900 ft to 1,030 ft, a difference of 130 ft: gently rolling country.
Under the town center the state geologic map shows Augusta Group (dolostone, limestone), from the Early Mississippian to Middle Mississippian, 331 to 359 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 64×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Dolostone, limestone. Age: Early Mississippian to Middle Mississippian, 331 to 359 million years old. About 10 mi of the line.
Shale, sandstone, siltstone. Age: Early Pennsylvanian to Middle Pennsylvanian, 307 to 324 million years old. About 5 mi of the line.
Dolostone, limestone, siltstone. Age: Early Mississippian, 347 to 359 million years old. About 4 mi of the line.
Dolostone, limestone. Age: Middle Mississippian to Late Mississippian, 324 to 347 million years old. About 2 mi of the line.
Shale, limestone, dolostone. Age: Late Devonian, 374 to 384 million years old. About 2 mi of the line.
Shale, siltstone. Age: Late Devonian, 359 to 374 million 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.
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 Grinnell reaches the Gulf of America by way of Sugar Creek, the North Skunk, the Skunk and the Mississippi: about 1,560 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Grinnell; river miles along the main stem, rounded.
Grinnell has a hot-summer humid continental climate. Summer afternoons average 83° in July; winter nights fall to about 10° in January. A normal year brings about 31 inches of snow; with the rain, that comes to about 38 inches of water.
On November 14 Grinnell normally reaches 47° and falls to 27°. The record high is 78°, in 1999; the record low −1°, in 1959. The most rain on this date, 1.21 in, fell in 1902.
Normals 1991–2020 and records since 1893 at Grinnell 3 SW, 2 miles southwest, 100 ft lower. NOAA, through RCC-ACIS.
