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Boxwork is an uncommon type of mineral structure, or speleogen (similar to a speleothem, but formed by erosion rather than accretion), occasionally found in caves and erosive environments.
Boxwork is commonly composed of thin blades of the mineral calcite that project from cave walls or ceilings that intersect one another at various angles, forming a box-like or honeycomb pattern. The boxwork fins once filled cracks in the rock before the host cave formed. As the walls of the cave began to dissolve away, the more resistant vein and crack fillings did not, or at least dissolved at a slower rate than the surrounding rock, leaving the calcite fins projecting from the cave surfaces.
Some of the most extensive boxwork deposits in the world are found in Wind Cave, Wind Cave National Park in South Dakota, USA. Other outstanding examples occur in Cody Caves, Cody Caves Provincial Park in British Columbia, Canada.