Caves

Cave formation is a striking example of how groundwater, not just surface rivers, reshapes the land. In limestone country, the slow chemical work of water over long periods carves out underground openings, and once those openings exist, they become sites where new rock material is deposited. The entire process hinges on one simple fact: limestone dissolves in slightly acidic water, and what dissolves can later be deposited elsewhere.

The right geological setting matters. Caves develop prominently where alternating beds of rocks — shales, sandstones, quartzites — are interlayered with limestones or dolomites. They also form where limestones occur as dense, massive, thick beds. In either case, the limestone must be present in sufficient quantity and purity for dissolution to be effective.

Water enters the ground by percolating downward, either through the rock materials themselves or through cracks and joints. Once underground, it moves horizontally along bedding planes — the natural surfaces between successive rock layers. It is along these bedding planes that the limestone dissolves. As the rock is eaten away, long and narrow to wide gaps develop, and these gaps are what we call caves.

The shape and extent of caves depend on the arrangement of the rock layers. Where limestone beds alternate with other rocks, or where the beds lie at different elevations, a maze of caves can form at various heights. The intervening non-limestone rocks act as barriers, so the dissolving water works along each limestone bed separately, producing a network of interconnected passages rather than a single simple cavity.

Caves typically have an opening through which cave streams are discharged — the underground water that did the dissolving eventually flows out. When a cave has openings at both ends, it is called a tunnel. So a tunnel is simply a cave that runs all the way through, open at two opposite sides. …