Sinkhole risk in Onslow County, NC: Sinkhole country

Sinkhole country

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

On soluble rock: 65%
At or near the surface: 65%
Buried under cover: 0%

What this means before you buy

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

That works out to roughly 498 of Onslow County's 762 square miles on soluble rock, 498 of them with the rock at or near the surface. With about 212,954 residents (279 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 260 map points about 1.7 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: the signs to look for and how an inspection works, and how these percentages are measured.

Counties around Onslow County

The closest county with no soluble rock mapped is Hyde County, NC, about 88 miles northeast. 6 of the 6 closest counties land in the same band, so this is a regional pattern rather than a local quirk.

Jones County, NC 20 mi northeast · 92% at surface
Duplin County, NC 27 mi northwest · 93% at surface
Pender County, NC 28 mi southwest · 89% at surface
Lenoir County, NC 34 mi north · 71% at surface
Craven County, NC 34 mi northeast · 69% at surface
New Hanover County, NC 45 mi southwest · 79% at surface

The rock underneath

The largest mapped soluble unit here is the River Bend Formation (limestone), Tertiary in age, covering about 40% of the county. Next is the Castle Hayne Formation; Comfort Member and New Hanover Member, undivided at 25%.

Against the rest of North Carolina

Onslow County ranks 19 of 100 in North Carolina for soluble rock at the surface, and sits above 91% of US counties on that measure.