Sinkhole risk in St. Clair County, AL: Pockets of it

Pockets of it

Soluble rock at the surface or under thin cover in part of the county.

On soluble rock: 45%
At or near the surface: 45%
Buried under cover: 0%

What to do about a house in St. Clair County

Check the parcel against your state survey sinkhole map, and ask for an inspection if cracks, depressions or sticking doors show up.

That works out to roughly 285 of St. Clair County's 632 square miles on soluble rock, 285 of them with the rock at or near the surface. With about 96,927 residents (153 per square mile), homes are spread across towns and farmland, and a mapped unit may or may not reach the developed parts. The figures come from 536 map points about 1.1 miles apart, so a pocket smaller than that can be missed or overstated.

Counties around St. Clair County

The nearest county rated sinkhole country is Talladega County, AL, about 25 miles south, close enough that the same rock may run under the county line. 4 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Blount County, AL 23 mi northwest · 23% at surface
Talladega County, AL 25 mi south · 60% at surface
Etowah County, AL 28 mi northeast · 46% at surface
Calhoun County, AL 28 mi east · 52% at surface
Jefferson County, AL 36 mi west · 21% at surface
Shelby County, AL 37 mi southwest · 30% at surface

The rock underneath

The largest mapped soluble unit here is the Conasauga Formation (limestone), Cambrian in age, covering about 13% of the county. Next is the Knox Group undifferentiated in part at 13%.

Against the rest of Alabama

St. Clair County ranks 16 of 67 in Alabama for soluble rock at the surface, and sits above 86% of US counties on that measure.