One interesting effect of the dissolving power of underground water in limestone regions is the development of caves or caverns. Most remarkable of all is Mammoth Cave, Kentucky, with its hundreds of miles of passageways and galleries. This marvelous work of nature is all a result of the action of underground water which has dissolved and carried away vast quantities of limestone. Echo River, which flows through the cavern, is still carrying on the work aided by various underground tributaries. The stalactites and stalagmites, which are so strikingly displayed in many caves, as at Luray, Virginia, in which water with carbonate of lime drips or oozes from the roof and, due mainly to evaporation, deposits the lime. Many wonderful and fantastic effects are thus produced. Where part of the roof of a cave is dissolved out, or falls in, a "sink hole" results. Where all but a portion of the roof of a cave or underground channel has fallen in, a natural bridge, like the famous one in Virginia, results, though natural bridges are also formed by other means. In concluding this chapter we shall briefly discuss hot underground waters, hot springs, and geysers. There are two well-known ways by which underground waters may become heated. One is by the movement of water downward into the normally heated portion of the earth, the rate of increase downward being, as above stated, 1 degree F. for about 50 to 60 feet. Water descending two miles would, therefore, attain a temperature of about 200 degrees F. In some regions such a temperature may be reached at depths considerably less. Such water (under pressure) taking a short course to the surface (forming springs) at a lower level would retain much of its heat taken up far below the surface. In regions where there are great down-folds of the strata (i.e., synclines), as in the central to southern Appalachians, conditions appear to be favorable for such warm or hot springs, as, for example, at Hot Springs, Virginia. A second cause of the heating of underground water is by the descent of surface waters into contact with masses of still hot igneous rock of relatively recent geologic age. In some such cases the water does not go more than some hundreds of feet down and when, under proper conditions, it returns to the surface hot and even boiling springs may result. [Illustration: Plate 7.--An Upbend Fold (_anticline_) in the Appalachian Mountain Strata Near Hancock, Maryland. The strata were deposited in horizontal layers upon the sea bottom, covering the region many millions of years ago in middle Paleozoic time. At the time of the Appalachian Mountain revolution, near the end of Paleozoic time, this and many other folds developed well below the surface. Removal of overlying material by erosion has laid bare the fold as we see it to-day. (_Photo by Russell, U. S. Geological Survey._)]