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Feature with Fire Cracked Rock Fire-cracked and/or burned rock appears primarily in sandstone, limestone, chert or quartzite raw materials. This rock has been culturally modified by burning only and appears as blocky pieces with irregular faces that may have fractured from the fire.  When heavily burned limestone becomes smooth, gritty and crumbles. These rocks usually break apart easily; they sometimes are charred but other than that, color will vary and is not a determining factor.  Pictured is a surface hearth consisting of large quantities of FCR and wood charcoal.  The rock helps to hold the heat and makes cooking more efficient.