Sinkhole risk in St. Charles County, MO: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 58%
At or near the surface: 14%
Buried under cover: 44%

What this means before you buy

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 324 of St. Charles County's 560 square miles on soluble rock, 80 of them with the rock at or near the surface. With about 423,726 residents (756 per square mile), most of the county is built up, so a mapped unit is likely to sit under somebody's street. The figures come from 441 map points about 1.1 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: why gypsum and salt move faster than limestone, and why the 44% under cover still counts.

Counties around St. Charles County

The nearest county rated sinkhole country is St. Louis County, MO, about 16 miles southeast, close enough that the same rock may run under the county line. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

St. Louis County, MO 16 mi southeast · 53% at surface
Lincoln County, MO 25 mi northwest · 0% at surface
St. Louis city, MO 25 mi southeast · 47% at surface
Warren County, MO 26 mi west · 42% at surface
Calhoun County, IL 26 mi north · 51% at surface
Jersey County, IL 27 mi northeast · 7% at surface

What St. Charles County is built on

The largest mapped soluble unit here is the Osagean Series (limestone), Early Mississippian-Osagean Series in age, covering about 25% of the county. Next is the Meramecian Series at 19%.

The county also lies over the Illinois Basin, a deep salt or gypsum basin. Deep beds rarely cause surface collapse on their own, but wells and mining into them have.

Against the rest of Missouri

St. Charles County ranks 67 of 115 in Missouri for soluble rock at the surface, and sits above 71% of US counties on that measure.