Rocky Midden High

By Bryan Heisinger

There is no denying that fire cracked rock (FCR) has a heavy presence in the research of the Ancient Southwest Texas Project. On a day to day basis, we sit on, trowel through, trip over, and often smell (strange.. I know) the enormous pile of FCR that fills Eagle Cave. Anyone who has worked with us long enough knows that we take our rocks seriously — and for good reason! By studying the FCR from our excavations we hope to address research questions about earth oven use and intensification in and around Eagle Nest Canyon.


Charles loves his burned rocks.

FCR is the by-product of rocks that have been used for cooking and heating purposes. A rock becomes “fire cracked” after it is exposed to intense heating/cooling and reuse in an earth oven or other thermal environments. Continuous episodes of thermal cycling cause the rocks to fracture into smaller, angular shaped pieces and once the rocks become too small to retain heat for cooking they are discarded in favor of newer/larger rocks. The accumulation of tossed FCR typically form in the shape of ring around the oven pit and in the case of rock shelters, they begin to form talus slopes. Ultimately, this ring or discard zone is categorized archaeologically as a burned rock midden.


In order to effectively study FCR and burned rock middens, reliable methods needed to be established for quantifying and categorizing the rock that we find during excavations. The Rocksort recording procedure was created as a way to document FCR using known size and attribute divisions that are common among earth oven literature and experimental studies. The size of FCR can tell us some information about the use-life of that rock and approximately how many times it was used for cooking purposes before it was discarded. The attributes of that particular rock (e.g. pitted limestone, roof spall, igneous/metamorphic rock) can help us determine the general source of the rock (e.g., uplands vs. canyon bottoms vs. within rockshelters).

It is important to note that we do not collect and record every rock that we encounter during excavation. Such a process would be extremely time consuming and would produce lots of repetitive data. Rather, we have been collecting and weighing FCR through selective column samples along our exposed profiles and other areas that we deem necessary or informative at our excavation sites. Through this selective Rocksort documentation, we will gain a representative sample of the varying densities and sizes of rock that are occurring at the sites we are investigating in and around Eagle Nest Canyon.

During the Rocksort process we split the FCR from a particular layer/strat or feature on a grid board (lines at 7.5 cm) into the following categories based on the maximum dimension : < 7.5 cm, between 7.5 -11 cm, between 11-15 cm, and > 15 cm (Fig. 1.). These rock size categories are based on the 20,000-odd FCR that were counted and measured as part of excavations at the Higgins site in San Antonio, directed by our very own Dr. Steve Black (see Higgins BRM).


Bryan and ASWT volunteer Kris Bobbit rocksorting an excavation layer in Eagle Cave.

Furthermore, this separation allows us to identify the stages of thermal fracture in the FCR and whether or not that particular layer/strat or feature being excavated is related to a discard event (e.g., small rocks <11 cm), a cooking event (e.g., larger rocks >11 cm), or some combination thereof.


A.) Predominately larger FCR >11  cm were found within cooking features; B.) Predominately smaller FCR 11< cm were found within discard zones.


Each square on the Rocksort board measures 7.5 cm and provides a speedy method for quickly measuring and grouping the hundreds of rocks that need to be sorted. After the rocks are sorted, they are photographed on the board and weighed according to their size and attribute class (Fig 2.).  This data is then entered on the excavation form and the sorted FCR is dumped into the back dirt piles.


Fig. 2: The ASWT Rocksort table (above) and the always famous Rocksort board (below).


The Big Picture

As mentioned earlier, the ultimate goal of our FCR documentation is to measure the amount of earth oven cooking that took place in Eagle Nest Canyon and other rockshelters and open sites in the Lower Pecos Region. However, you may be wondering how in the name of Einstein do we siphon our Rocksort data into something understandable?

Sparing you the math, we can take this data and calculate the approximate volume, FCR mass, and FCR density  of the burned rock middens in and around Eagle Nest Canyon. This data – with the help of radiocarbon dating – can tell us when and how much earth oven cooking took place at each site in the canyon over time. Additionally, we can compare the Eagle Nest Canyon FCR data with that from previous ASWT projects along the Devils River to give us an idea of the amount of earth oven intensification that occurred across the Lower Pecos landscape over time. Pretty cool right?

What it all boils down to is having the ability in the future to be able to estimate the number of earth ovens at different sites with minimal excavation. We hope to be able to not only compare earth oven features across the Lower Pecos but possibly North America.