Sinkhole risk in St. Louis County, MO: Sinkhole country

Sinkhole country

Soluble rock at or near the surface across much of the county.

On soluble rock: 71%
At or near the surface: 53%
Buried under cover: 18%

What to do about a house in St. Louis County

Get a sinkhole or geotechnical inspection before buying, building or adding weight to a foundation.

That works out to roughly 362 of St. Louis County's 509 square miles on soluble rock, 271 of them with the rock at or near the surface. With about 992,929 residents (1,953 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 456 map points about 1.1 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around St. Louis County

The closest county with no soluble rock mapped is Macoupin County, IL, about 51 miles northeast. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

St. Louis city, MO 11 mi east · 47% at surface
St. Charles County, MO 16 mi northwest · 14% at surface
Jefferson County, MO 27 mi south · 98% at surface
Monroe County, IL 29 mi southeast · 6% at surface
St. Clair County, IL 30 mi southeast · 0% at surface
Jersey County, IL 31 mi north · 7% at surface

Which formation it is

The largest mapped soluble unit here is the Meramecian Series (limestone), Late Mississippian-Meramecian Series in age, covering about 34% of the county. Next is the Osagean Series at 15%.

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. Louis County ranks 55 of 115 in Missouri for soluble rock at the surface, and sits above 88% of US counties on that measure.