. Elementary physical geography . lustrate the formation of caverns. work of underground water is not confined to the surfacelayers, but extends to considerable depths in the , from our present standpoint, the most importantchanges are those which are produced nearly at the surface,and these are again referred to below. The Formation of Caverns. — Limestone is one of the mostsoluble of the rocks ; and in many of the regions where thisexists, the solvent action of underground water goes so faras to actually dissolve cavities in the strata (Fig. 106). Itsinks into the ground through
. Elementary physical geography . lustrate the formation of caverns. work of underground water is not confined to the surfacelayers, but extends to considerable depths in the , from our present standpoint, the most importantchanges are those which are produced nearly at the surface,and these are again referred to below. The Formation of Caverns. — Limestone is one of the mostsoluble of the rocks ; and in many of the regions where thisexists, the solvent action of underground water goes so faras to actually dissolve cavities in the strata (Fig. 106). Itsinks into the ground through depressions, or sink holes(Fig. 107), and passes along planes of weakness, which itenlarges by solution; and in some cases, this undergroundwater assumes the form of true subterranean rivers, which DENUDATION OF THE LAND. 227 are sometimesseveral miles inlength. The cav-erns (Fig. 106)thus formed, arevery irregular;and some, suchas the MammothCave of Ken-tucky, and Lu-ray Cave, havebeen exploredand opened totourists: but there. Fig. 107. A sink hole in a limestone region. are thousands
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