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Feature with Fire Cracked Rock Fire-cracked or burned rock, often called FCR, has been culturally modified by burning only. It appears as blocky pieces with irregular faces that may have fractured from the fire. FCR in western Wisconsin is often sandstone, limestone or dolomite, chert, or quartzite. When heavily burned, limestone becomes smooth, gritty, and crumbly. These rocks often break apart easily; sometimes they are charred, but otherwise, color can vary. The photo shows a hearth area with wood charcoal and large quantities of FCR, from a site in Vernon County. The rock would have helped to hold the heat and make cooking more efficient.