Calcareous rocks, like limestone and marble (a rock modified by heat and pressure), are made mainly by calcite, a mineral form of the calcium carbonate. The calcite is insoluble in pure water, but the rain water contains carbon dioxide, coming from air or soil, and forms a weak acid that react with the calcite, resulting a soluble salt, calcium bicarbonate, which is transported as a solution.
Another trait of the calcareous rocks is given by their strong bonding. Vertical bonds formed when the original deposits dehydrated and contracted. Horizontal bonds are sheaths interposed between the sedimentary deposits that later transformed in rocks. These joints allow the table waters to pass through the rocks that otherwise would be solid and even impermeable.
The most spectacular cave systems appear in the calcareous rocks, in the seeping of the table waters through the rocks. They gradually enlarged the cracks, turning them into vertical chimneys, passages and vast opened chambers. When the level of the table water rises, the caves are flooded. When the level lowers, a labyrinth of tunnels and grottoes appears. The table waters go out at the base of the calcareous rocks, where the downward movement of the water is stopped by an impermeable, solid rock. That's why the base of a calcareous deposit is marked by a series of underground springs. In many cases, the abrupt slopes of mountain gorges may have formed following the collapse of caves.
Caves with rugged decors form a karstic landscape. In many cases, abrupt ravines follow the vertical joints. The surface of the calcareous rocks is often interrupted by narrow holes or orifices. The orifices formed in the joint areas of the rocks can widen in deep ravines swallowing springs.
Because of the water seeping through joints and cracks, calcareous surfaces or pavements are usually dry. Calcareous rocks are delimited in "cleavage blocks" and separated by large ravines. At major joints, the rock is dissolved, forming funnel-like cavities or larger sinkholes, called dolines. They can be 100 m (330 m) in section and can reveal subterranean rivers that run from one cave into another.
Many calcareous surfaces contain the valleys of drought rivers, and many valleys could have been carved during the Ice Age, when the soil was frozen and the water could not seep through the calcareous soil, digging valleys on the surface.
Drought valleys can also appear through erosion. With the sinking of the surface, table waters sink too, and the surface rivers go dry.
In cold areas, waters come especially from the melting of the snow and weep the encountered sediments, leaving behind a barren, rocky pavement.
Warm regions are dominated by wide valleys called polje, connected through abrupt slopes. They can have surfaces up to 250 square km and form temporary lakes in the winter, when the water cannot be drained through the cave system.
In wet tropics, karst areas can form abrupt hills rising over flat plateaus, the so called "karst towers". Adjacent dolines can widen until flat terrains between them disappear, leaving behind a series of isolated cliffs, called karst cones.
The most common limestones are:
_chalk, one of the purest types, made by the microscopic remains of plants and animals;
_coralline rocks, limestone layers made by the external skeleton of the coral polyps;
_dolomite, with a high content of calcium and magnesium carbonate;
_fossil limestone, containing fossils, like shells combined through calcite cement;
_oolite limestone, containing calcite particles formed when calcite covers small particles like sand or shells. Pisolitic limestone contains larger particles;
_travertine, a compact limestone deposit, like the stalactites and stalagmites from caves or around springs and geysers. Tufa is a spongy type of travertine.