The Big Rock 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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−6° on Nov. 12, 2019, the coldest reading for this week of the year since 1996. Next is 3° in 1996.
Big Rock’s center stands about 700 ft above sea level. Within three miles the land runs from 660 ft to 810 ft, a difference of 150 ft: gently rolling country.
Under the town center the state geologic map shows Scotch Grove Formation (dolostone), from the Silurian, 431 to 440 million years old.
The section below crosses 4 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. Age: Silurian, 431 to 440 million years old. About 17 mi of the line.
Dolostone, mudstone. Age: Silurian, 425 to 434 million years old. About 5 mi of the line.
Shale, sandstone, siltstone. Age: Early Pennsylvanian to Middle Pennsylvanian, 307 to 324 million years old. About 2 mi of the line.
Dolostone. Age: Late Ordovician to Silurian, 434 to 447 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 Big Rock reaches the Gulf of America by way of Rock Creek, the Wapsipinicon and the Mississippi: about 1,520 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Big Rock; river miles along the main stem, rounded.
Big Rock has a hot-summer humid continental climate. Summer afternoons average 84° in July; winter nights fall to about 14° in January. About 36 inches of rain fall in a normal year.
On November 14 Big Rock normally reaches 49° and falls to 30°. The record high is 72°, in 2025; the record low 3°, in 1996. The most rain on this date, 0.25 in, fell in 2020.
Normals 1991–2020 and records since 1996 at Davenport Municipal Airport, 16 miles southeast. NOAA, through RCC-ACIS.
