Fire-Cracked Rock, part 1

Interior of Fire-Cracked Rock Old Colchester Park & Preserve Fairfax, Virginia

Interior of Fire-Cracked Rock
Old Colchester Park & Preserve
Fairfax, Virginia

Exterior of Fire Cracked Rock Old Colchester Park & Preserve Fairfax, Virginia

Exterior of Fire Cracked Rock
Old Colchester Park & Preserve
Fairfax, Virginia

by Sheila Koons – Lab Archaeologist & Lithic Specialist

One of the most common artifacts recovered from Old Colchester is fire-cracked rock (also known as FCR), particularly from the prehistoric areas. Not all cracked rock is indicative of human activity. So then how can we tell if the rock was cracked naturally or by purposeful humanly-controlled fire?

First, it is important to be able to identify fire-cracked rock. Fire-cracked rock is usually quartzite, sandstone, or granite, and is found fractured and split (McDowell-Loudan 1983:24). The majority of fire-cracked rock at Old Colchester happens to be quartz which is a raw material that is abundant in the area. FCR typically retains the exterior or cortex of the cobble, the fractured interior surfaces are crenulated, and the fracture edges are jagged and angular. The cortex may take on red, pink, or black color or it may not change color at all. When quartz is heated, the increase in temperature causes the internal development of fissures and water loss which in turn cause an increase in porosity and a subsequent decrease in density. This finally leads to the expansion of the lithic material (Audley 1921:209 cited in Ibid.). Then the rock will fracture or at least develop fissure lines. On the other hand, quartz can also fracture when cooling down after heating. This sort of FCR will have more cortical or surface damage such as pitting and this will yield another artifact commonly known as a potlid. If a rock is heated or cooled too quickly it will spall off in irregular segments.

Next it is important to know the context within which the burned rock was found. When recovered in large abundance, fire-cracked rock is typically associated with hearths and activities that took place around the hearth (e.g. roasting, steaming, sweatlodges, and potprops) (Akins 1985). As Barbetti stated,”Demonstrating that fire was used at an archaeological site is a two-step process. One must first find the evidence and show that fire was present. It is then necessary to demonstrate that it was associated with human activity” (1986:771). One of the most common causes of natural fire is lightening (Westbroek et al. 1993). Burnt tree stumps and brush fires seem to leave similar impressions as camp fires or hearths (Clark and Harris 1985; Bellomo 1993, 1994). One archaeologist performed actualistic studies of fire at FxJj 20 Main, Koobi Fora, Kenya (Bellomo 1993, 1994). He was particularly interested in recreating what happens in the ground during and after campfires, tree stump fires and grass fires. Using a combination of archaeomagnetic studies and several other studies which measure different aspects of sediment magentism, the archaeologist was able to test for any changes in the magnetic mineralogy from fires. His temperature data showed that campfires burn at higher temperatures than tree or grass fires (544). From this, he was able to deduce that sediments under such humanly-controlled fires (as campfires or hearths) would display a higher change in their magnetism. According to his temperature data, campfires could get as hot as 600oC and maintain a constant 400°C for 1.5-3 hours (533). Neither grass or tree stump fires came close to maintaining such temperatures. Additionally, Bellomo showed that in profile, camp fire and tree stump features were actually completely different (Ibid). Whereas multiple campfires (in one spot) burned a basin-like shape about 15cm below ground surface, the tree stump burned a hole straight into the substratum (Ibid.).

At Old Colchester, there were no hearth features identified but we have so much FCR it begs further questions. Additional site formation processes may provide answers. The features may be deflated or perhaps destroyed by historic bulldozing activity. My next installment will be about the difference between fire-cracked rock and thermal pretreatment of rock.


Barbetti, M. (1986). “Traces of Fire in the Archaeological Record Before One Million Years Ago?” Journal of Human Evolution 15: 771-781.

Bellomo, R. V. (1993). “A Methodological Approach for Identifying Archaeological Evidence of Fire Resulting from Human Activities.” Journal of Archaeological Science 20: 525-553.

Bellomo, R. V. (1994). “Methods of determining early hominid behavioral activities associated with the controlled use of fire at FxJj 20 Main, Koobi Fora, Kenya.” Journal of Human Evolution 27: 173-195.

Clark, J.D. and Harris, J.W.K. (1985) Fire and its roles in early hominid lifeways. The African Archaeological Review 3:3-27.

McDowell-Loudan, E. (1983) “Fire-Cracked Rock:Preliminary Experiments to Determin its Naturea and Significance in Archaeological Contexts. The Chesopiean 21(1):20-29.

Westbroek, P., M. J. Collins, et al. (1993). “World archaeology and global change:Did our ancestors ignite the Ice Age?” World Archaeology 25(1): 122-133.


About cartarchaeology

We are the County Archaeological Research Team, part of the Archaeology and Collections Branch, Resource Management Division, Fairfax County Park Authority. We are tasked with understanding and managing the cultural resources on Park land throughout Fairfax County.
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