A sinkhole (also called a doline) is a depressed area usually formed by solution of surficial bedrock or collapse of underlying caves. The surface expression of a sinkhole is typically a conical depression or area of internal drainage. Sinkholes range in size from several square yards to hundreds of acres. They may be quite shallow or may extend hundreds of feet deep. Sinkholes are places where there is rapid recharge (replenishing) of groundwater from the surface and, therefore, are areas of potential groundwater contamination. For this reason, managing surface water and waste disposal in sinkhole-prone areas are important to maintaining good groundwater quality.

The diagrams below conceptually illustrate the stages of sinkhole formation. Actual conditions in nature may be very different than those illustrated. For instance, the rock and soil layers may be thicker or thinner, the fracture and cave passage may be larger or smaller, and the surfaces are likely to be much more irregular in shape.

Stage 1
For a sinkhole to form there must be an opening in the bedrock surface that allows overlying soil to move downward into a cave passage. This stage illustrates a solution-widened fracture in the bedrock choked with soil.

Stage 2
Soil that collected in the cave passage in Stage 1 has been carried away by flowing water.

Stage 3
Soil that collected in the fracture or bedrock opening collapses into the cave or is washed into the cave by water movement from the soil into the cave.

Stage 4
Additional soil movement or collapse causes a void to form at the bedrock surface.

Stage 5
The void enlarges and moves upward in the soil profile, a process known as stoping.

Stage 6
Eventually the void enlarges until only a thin layer of soil remains at the surface.

Stage 7
Finally the thinned soil roof can no longer support itself and creates a surface collapse that may or may not choke the hole in the bedrock. Typically the initial appearance is a steep-sided hole at the surface several feet deep with a floor of soil that used to be at the surface.

Stage 8
If the bedrock throat of the sinkhole remains plugged with the collapsed soil, the surface hole may fill with other eroded soil.

In some instances the unstable, steep-sided surface hole may widen into a conical depression, like the upper portion of an hour glass. If the throat of the sinkhole remains open, surface water will drain readily. If the throat becomes plugged with soil, water may pond temporarily or permanently in the depression, forming a sinkhole lake. The entire process may repeat itself by starting over.