Fire-cracked rock

In archaeology, fire-cracked rock is rock of any type that has been altered and split by deliberate heating. It is a feature of many archaeological sites, particularly in the south-central United States. In many cases, FCR results from stones being used to line hearths or from being heated to provide a longer-lasting heat-source (similar to historic water bottles). In other cases, FCR results from stone being used to heat water; this occurs when the stones are heated and dropped directly into containers made of skin or pottery, thus boiling the water. Central Texas in the United States is well-known archaeologically for its burnt-rock middens[?], piles of FCR -- often exceeding several tons of material -- which resulted from the wholesale processing of oak acorns during prehistoric times.