Sinkhole risk in St. Mary's County, MD: 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: 15%
At or near the surface: 15%
Buried under cover: 0%

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 52 of St. Mary's County's 359 square miles on soluble rock, 52 of them with the rock at or near the surface. With about 116,469 residents (325 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 442 map points about 0.9 miles apart, so a pocket smaller than that can be missed or overstated.

Two things to read before deciding: how these percentages are measured, and the warning signs that apply anywhere.

Counties around St. Mary's County

The nearest county rated sinkhole country is Clarke County, VA, about 100 miles northwest. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Westmoreland County, VA 17 mi southwest · 20% at surface
Calvert County, MD 21 mi north · 47% at surface
Richmond County, VA 22 mi southwest · 23% at surface
Northumberland County, VA 27 mi south · 12% at surface
Essex County, VA 29 mi southwest · 42% at surface
Dorchester County, MD 30 mi northeast · 0% at surface

Which formation it is

The largest mapped soluble unit here is the Chesapeake Group; Choptank Formation (sand), Miocene in age, covering about 12% of the county. Next is the Chesapeake Group; Calvert Formation at 3%.

Against the rest of Maryland

St. Mary's County ranks 7 of 24 in Maryland for soluble rock at the surface, and sits above 71% of US counties on that measure.