A sinkhole is a natural depression or hole in the Earth's surface which may have various causes. Most natural sinkholes are caused by the chemical dissolution of water soluble carbonate rocks or gypsum. Sinkholes may vary in size from 1 to 600 m both in diameter and depth. Sinkholes may be formed gradually or by sudden collapse, and are found worldwide in karst areas. The term "sinkholes" has also been used to describe ground subsidence caused by anthropogenic activities, for example the collapse of old mine workings close to the surface, also known as abandoned mine openings.
In Nova Scotia most natural sinkholes or karst topography are formed over areas where Windsor Group gypsum occurs in the near surface. The geological boundary between the gypsum- and salt-bearing Windsor Group and the underlying sandstones and shales of the Horton Group or older basement rocks is particularly prone to sinkhole development.
In areas of sinkhole development the primary geohazard is related to sudden catastrophic subsidence due to collapse of cavities in the bedrock created by the dissolution of the gypsum and/or limestone. Caution should be exercised in these areas prior to construction of buildings or roads.