Sinkhole risk in Boulder County, CO: 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: 4%
At or near the surface: 4%
Buried under cover: 0%

The next step for one property

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

That works out to roughly 30 of Boulder County's 726 square miles on soluble rock, 30 of them with the rock at or near the surface. With about 330,262 residents (455 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 363 map points about 1.4 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 how these percentages are measured.

Counties around Boulder County

The nearest county rated sinkhole country is Dawes County, NE, about 216 miles northeast. 2 of the 6 closest counties land in the same band; the ground changes quickly around here.

Gilpin County, CO 18 mi southwest · 0% at surface
Broomfield County, CO 21 mi southeast · 0% at surface
Clear Creek County, CO 32 mi southwest · 0% at surface
Denver County, CO 36 mi southeast · 0% at surface
Jefferson County, CO 37 mi south · 2% at surface
Grand County, CO 38 mi west · 6% at surface

Which formation it is

The largest mapped soluble unit here is the Lykins Fm and Lyons Sandstone (sandstone), Triassic-Permian in age, covering about 3% of the county. Next is the Colorado Group at 1%.

3% of the county sits on gypsum or salt, which dissolves in years rather than millennia. That share matters more than its size suggests.

Against the rest of Colorado

Boulder County ranks 20 of 64 in Colorado for soluble rock at the surface, and sits above 62% of US counties on that measure.