This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The chemically formed cave deposits are due to the solution and redeposition of carbonate of lime. Caves are very generally found in limestones, and the percolating waters which make their way through the roof of a limestone cavern always have more or less CaC03 in solution. A drop of such water, hanging from the cavern roof, will lose some of its C02, upon the presence of which the solubility of the CaC03 depends, and deposit a little ring of the lime salt. Successive depositions will lengthen the ring to a tube, and then the tube will be built up by layers on the inner side, until it becomes a cone. At first, the deposit is white, opaque, and very friable, crumbling at a touch, but repeated depositions fill up the interstices of the porous mass and convert it into a hard, translucent stone, which assumes a crystalline structure through the development of calcite or ara-gonite crystals. The masses, thus formed, that depend from the roof of the cavern, are called stalactites. After hanging for a time from the roof, the drop of water falls to the floor of the cave, and there, in similar fashion, deposits a little layer of CaCO3, which gradually grows upward into a cone.
This is a stalagmite, and differs from the stalactite only in the fact that it grows upward from the floor, instead of downward from the roof. The stalagmite is, of course, exactly beneath the stalactite, and as long as the water continues to follow the same path, the two cones are steadily, though very slowly, increased both in height and thickness, until they meet, unite, and form a pillar extending from floor to roof of the cavern.
These deposits form the most curious and beautiful features of limestone caverns. The stalactites assume all manner of shapes, determined by the way in which the water trickles over them, and the abundance or scantiness of the water supply. Fantastic and beautiful shapes of every description, fringes of crystal spar, and curtain-like draperies hang from the roof and cover the walls of the chambers, while grotesque shapes rise from the floor, which is itself often a solid mass of the same deposit, and the pillars, once formed, are ornamented with every variety of fringe and sculpture. The constancy of the paths by which the water descends through the roof of the cavern, insures that the process shall continue uninterruptedly for very long periods of time. The Luray Caverns of Virginia are famous for the bizarre beauty of their formations, but limestone caves everywhere have more or less of the same deposit to show.
This process may be readily observed in any masonry arch, through which rain-water percolates, as a bridge, for example. The lime of the mortar is converted, in course of time, by contact with moist air, into CaC03, and this again is partially dissolved by the rain. When the rain-water trickles through the arch, it leaves icicle-like deposits, or thin sheets of calcareous matter, fringing the under side.
In a cave, it frequently happens that angular fragments fall from the roof and are cemented into a breccia by deposits of stalagmite. In caves connected with the surface by openings, sand and gravel, or fine soil and loam, are washed in by streams, or by the rain, and form the characteristic deposit known as cave earth. In ancient caverns, no longer subject to this wash, the whole deposit of earth may be sealed in by a covering of stalagmite. Cave earth has, in many instances, yielded great quantities of bones, which were washed in with the earth, or dragged in by the carnivorous animals which inhabited the cavern. The Port Kennedy cave in Pennsylvania is almost filled up by the bones of extinct animals which were washed into it, and many such cases are known, especially in Europe.