Sinkholes
A sinkhole is a naturally occurring, roughly circular depression in the land
surface, formed most commonly in areas of limestone bedrock. Limestone is a
type of rock composed entirely of the highly reactive mineral calcite (CaCO3),
which readily dissolves in the presence of slightly acidic ground water. In areas of
humid climate, rain water percolates downward through the soil cover into
openings in the limestone bedrock, gradually dissolving the rock matrix. Void
spaces in the subsurface will eventually form, ranging from microscopic to cavern
size.
 | | Aerial picture of Florida sinkhole lakes. |
In most areas of the southeastern United States, the limestone bedrock is not
directly exposed at the surface, but is covered by a variable thickness of sand, silt
and clay. This overburden may bridge subsurface cavities for long periods of
time. Eventually a catastrophic collapse of the overburden into the subsurface
cavity may occur, and a sinkhole is formed. This type of sinkhole is known as a
cover collapse sinkhole.
 | | Huge cover collapse sinkhole in Winter Park, Florida. Use the trucks in the foreground for scale. |
|  | | Another view of the Winter Park, Florida sinkhole. |
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In North Carolina, sinkholes are common features of the outer coastal plain in
areas where the Castle Hayne or River Bend Formations occur at or near the surface. Most NC sinkholes become flooded and appear as small to medium sized circular lakes.
They can be distinguished from non-sinkhole lakes by the absence of any
outflow drainage and lack of relationship to surface drainage systems.
 | | Stippled area of North Carolina coast is underlain by the Comfort Member of the Castle Hayne Formation or the River Bend Formation which are known for their solution features. |
|  | | Portion of USGS topographic map near Catherine Lake, Onslow County where several sinkhole lakes are present. |
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A cover collapse sinkhole is just one end of the sinkhole spectrum. At the
opposite end of the spectrum is the cover subsidence sinkhole, formed where
overburden is relatively thin (a few feet to tens of feet). In this setting, as
subsurface solution occurs, the land surface gradually subsides into the void
space below, since it lacks the cohesiveness to form a significant "bridge" across
the void. Cover-subsidence sinkholes are often mistaken for other land subsidence
features, since they do not form in as spectacular a manner as the cover-collapse
sinkhole. One common indicator of this type of sinkhole is the formation of
cracks in nearby buildings or in roads.
Under natural conditions, sinkholes usually form rather slowly, over the course of
many years. However, some human activities can trigger abrupt sinkhole
formation, or accelerate processes that have been going on for a long time.
Activities such as dredging, diversion of surface drainage systems, or pumping of
ground water can accelerate the natural growth of sinkholes.
 | | Large North Carolina sinkhole formed as a result of nearby pumping of ground water. |
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