contact of groundwater with rock introduces other impurities which hasten solu-
tion. Solution rates are fastest where groundwater is not stagnant and passes over
interior rock surfaces without becoming saturated and slowing solution. Rocks
containing calcium are particularly vulnerable so that the plentiful limestone,
dolomite, halite, and gypsum exposed near Earth’s surface are candidates for
forming karst. Aluminum-, iron-, and silicon-based rocks are much more resilient
to erosion. Bedding, jointing, and faulting structures can be vital because they pro-
vide avenues of least resistance through which the water can infiltrate vertically
and move laterally. Vast amounts of rock can be removed via solution so that over
time the near-surface bedrock takes on a “swiss-cheese” configuration with all
materials eventually dissolved.
An irony of active karst landscapes is the lack of surface streams in rainy
climates. The “plumbing” is dominant, and underground drainage is the path taken
by rainfall. Dry streams, which flow only during extreme rain events, are common,
and some streams are actually swallowed into the ground. Such flows have been
traced many kilometers to outlets in springs and streams. The water is not channeled
into an individual underground stream but flows slowly through myriad intercon-
nected tunnels. This underground water is dissolving materials to create all manner
of cavities, some of which are expressed as landforms on the surface.
There are many types of distinct landforms in karst topography. The most
common of these is a sinkhole (doline). Sinkholes are underground cavities that
have had their roofs collapse and are open to direct entry of water. They are
steep-sided with inward slope angles of 20–30 percent and range from a few
meters to many kilometers across. Prodigious numbers of sinkholes exist in some
karst landscapes such that surface activities are severely limited. Sinkholes can be
either dry or beneath the level of the water table and therefore filled with water.
A uvala is a series of interconnected sinkholes forming a steep-sided valley with
no outlets.
As the landscape “rots” from within, the sinkholes and uvalas enlarge and leave
behind some steep-sided erosional remnants. The most common of these are hay-
stack hills (magotes), and they are quite visible in parts of western Kentucky,
Cuba, and Puerto Rico with heights averaging up to 25 m with diameters of up
to 200 m. Tower karst is, perhaps, the most dramatic of the karst forms. Haystack
hills are nubs compared to the grand spires of tower karst. These are the erosional
remains of much higher landscapes and represent landscapes in late stages of
topographic development. Through the process of solution over eons, the rock
has been left in steep-sided cones and these are undercut and further steepened
into towers. Some of these towers can exceed 300 m in relief and are riddled with
cavities. Perhaps the most phantasmagorical landscape on the planet is the karst
towers along the River Li in southern China. The juxtaposition of the lowlands194 Karst