Rimstone

Rimstone is a type of speleothem, a mineral formation that occurs within caves, primarily made of calcium carbonate. The name derives from the French word "rimstone," meaning wall or castellated margin, reflecting the formation’s resemblance to a scalloped or terraced rim. Rimstone formations develop over long periods when mineral-rich water slowly deposits calcite in a rim-like pattern around a cavity or pond in a subterranean environment. The water typically originates from rainfall that infiltrates the soil, dissolves limestone or other carbonate rocks, and then seeps into fractures within cave roofs or walls.

There are two main varieties of rimstone. Pseudo‑rimstone, also known as a “pseudo‑stalactite,” forms as a pressure rim along the edge of a pond or pool of water. As the water evaporates, calcite precipitates at the rim, creating a smooth, often concave surface. True rimstone, or rimstone dam, builds in a more constricted shape and can form tall, discrete pillars or sheets of calcite that resemble miniature walls or terraces. The rate of formation is normally very slow, with thicknesses of a few millimetres accumulating each year.

Rimstone structures are most prominently found in karst landscapes, especially in limestone caves in regions such as New England, the Ozarks, and parts of the Appalachian Mountains. They are valued by speleologists and geologists as important indicators of past hydrological conditions and humidity levels within caves. While many rimstone features are fragile and susceptible to damage from human traffic, some well-preserved systems are protected within national park cave and karst preserve boundaries.

Because of their delicate nature, many cave management guidelines recommend limiting visitors to a minimum number of passes across rimstone-occupied sections, using rope restraints in particularly sensitive areas, and ensuring that any maintenance activities are conducted by trained personnel. Rimstone is also an educational tool, illustrating geochemical processes such as calcite precipitation and the interaction between groundwater and cave morphology.