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SUBSIDENCE
Depression the result of subsidence. By definition,
subsidence is the very slow to rapid sinking or settling of
the land surface.
Subsidence can be the result of natural causes.
Some type of carbonate rock underlies topography
containing numerous natural depressions, known as
sinkholes. The topography is known as karst topography.
Limestone and dolomite, both carbonate rocks, are soluble
and susceptible to chemical weathering. Chemical
weathering produces void spaces (very very small to
cavernously large). Sinkholes result when enough
"support" has been removed from the carbonate layer. The
surface then collapses into the void space, producing a
sinkhole.
GEOL g406 Environmental Geology S. Hughes, 2003