|What can fire-modified rock tell us?|
Fire-modified rocks are cobbles and pebbles that have been used by humans in the past during cooking or heating activities. Rocks that are found to be from a hearth or campfire may have been used to form a boundary to keep the fire from spreading. Rock also retains heat longer than charcoal or ash, so rock was often incorporated into earth ovens or stove-like features to bake foods that required longer cook times. When heated, the rock often becomes discolored (reddened or blackened) and may crack. These fire-modified rocks are often found during archaeological excavations, sometimes in the same form in which they were originally used.
Fire-modified rock feature found at the Gee Creek archaeological sites
The number of pieces of fire-modified rock, their average weight, and their density were calculated for each of the features found at the Gee Creek sites. These basic analyses were used to compare feature assemblages and to make some general statements about the type of rock being used. For example, larger assemblages of fire-modified rock suggest the presence of larger features, which may imply that a greater amount of food was processed in the feature. Assemblages with a high average weight per rock may represent a feature composed of rock being used for the first time rather than reused. In oven features, longer term baking results in smaller and lighter spalls relative to short-term baking. This suggests that the greater the average weight per rock in an assemblage, the shorter the cooking time.
The characteristics of the cracking or fracturing of each assemblage also were determined. These indices included the ratio of cobbles to spalls in an assemblage, the percent of cracks or spalls observed on the cobbles, and the mean size of the fire-modified rock found in each assemblage. If a rock feature is reused, a certain amount of maintenance is expected, including the removal of small fragments of rock, charcoal, and other debris from the feature. A greater ratio of spalls to cobbles in an assemblage suggests that a feature may have been less maintained. In oven features, a greater amount of cracking and spalling suggests a relatively longer heating period. Longer term baking also results in smaller and lighter spalls relative to short-term baking. This suggests that average spall size in an assemblage may indicate relative cook times.
The color of the fire-modified rock and surface area affected were analyzed to determine the length of cooking, with greater surface area affected but duller reddening (low chroma numbers) associated with longer cook times. These analyses provided the archaeologists with important information about how the rock oven was used, for how long, and for what types of food.
Ozbun, Terry L., Judith S. Chapman, Maureen N. Zehendner, Eric E. Forgeng, and
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The information provided on this website is modified from the manuscript Archaeological Data Recovery for the I-5/SR 502 Interchange Project, by Michele Punke, Terry Ozbun, Jo Reese, and Brian Buchanan (2009). Archaeological Investigations Northwest, Inc. Report No. 2273. Prepared for Washington Department of Transportation Southwest Region, Vancouver, Washington.