April 3, 2012

Quercus spp. - Acorn


Acorns were eaten by native eastern North American people in most if not all regions where oak trees grew. Archaeobotanists usually find small fragments of carbonized acorn shell: these can be identified to the genus level. Carbonized acorn nutmeats are occasionally recovered by flotation but may be difficult to recognize unless a good portion of the outer surface is present. Dry rockshelter sites have yielded desiccated acorn shell in midden debris, along with a few caches of whole acorns stored in large bags or prepared pits (Hilliard 1986).


Flotation-recovered archaeobotanical samples from forested parts of eastern North America are likely to include carbonized acorn shell fragments. Because most acorn shell pieces are smaller than 2.0 mm, they should be counted an reported down to the 1.4 mm or even 1.0 mm cut-off point, and the smallest sieve size from which acorn shell was counted should be stated in each report.

Figure 2. Cross section of modern Q. alba L. acorn shell. The actual specimen  is 0.6 mm to 0.8 mm thick.

Acorn shell (pericarp) is usually thinner than 1.0 mm and frequently as thin as 0.4 to 0.6 mm. It consists of two distinct layers, except in the region under the cap where only one layer is discernible (Figures 1, 2, and 3). The outer layer is usually the thicker, varying between ca. 0.2 and 0.8 mm, with a delicate, palisaded pattern in cross-section, the lines running perpendicular to the surface. The surface of the outer layer is smooth and lacks patterning at low magnification (10-50x). It is not unusual, however, for tiny, roundish or oval-shaped holes to be visible in the exterior surface of carbonized pieces.

Figure 3. Rough sketch of cross-section of acorn shell, illustrating inner and outer layers.
The inner layer of acorn pericarp consists of a series of papery sheafs attached to the harder outer coat, producing a laminated cross-sectional pattern with layers running at right angles to the fine, parallel lines of the palisaded outer layer. Due to the fragility of the inner layer, some parts of it may not survive the rigors of time. When the inner layer of a fragment of acorn shell is facing the inexperienced analyst through the microscope, it may be confused with wood xylem. Therefore, each piece should be flipped over, and the cross section should be examined, as well. Fragments of carbonized acorn shell tend to be flat, but they may retain their original convex shape or even curl around so that the exterior surface is concave. Recognition of archaeobotanical acorn shell is difficult when only one layer is preserved, and it is best to omit  dubious fragments from the tally.

Under special circumstances, archaeological acorn may be identifiable to subgenus or even species. Uncarbonized specimens with caps (cupules) and tips offer the greatest potential. When entire cotyledons (seed halves) are recovered, variability in size may indicate whether more than one species is represented. Small carbonized shell fragments, however, contain few, if any taxonomic clues. Members of the red oak subgenus have an extra inner shell lining that is silky-hairy. Species in the white oak subgenus, lacking this layer, have smooth inner shells (Hilliard 1986; Steyermark 1963). Presence of the fuzzy lining obviously means a member of the red oak group was deposited, but its absence from a carbonized fragment could be due to lack of preservation. My impression is that the silky-hairy layer rarely survives, so that most charred fragments are recognizable only to genus.

Fragmentary acorn shell may be confused with chestnut (Castanea dentata (Marshall) Borhk), chinquapin (Castanea pumila (L.) Miller), pecan (Carya illinoensis (Wangenh.) K. Koch), and other thin hickory nutshell. Chestnut, chinquapin, and acorn shell are so similar that, in areas where more than one was common, it might be best to use an inclusive “Fagaceae” category. Pecan shell lacks the two structurally distinct layers of Quercus and Castanea shell, and only the last two have the “palisaded” effect of their outer layer. The portion of acorn shell covered by the cap is denser, lacks two layers, and has no palisaded structure, so it is the easiest to confuse with thin hickories such as pecan.

Archaeological Distribution

I can think of no sub-region in the temperate Eastern Woodlands where flotation recovery has failed to yield acorn shell. In their study of plant food remains from the Southeast, Yarnell and Black (1985) demonstrate that relative frequencies were highest during the Early Archaic (7500-6500 B.C.) and Early Woodland periods (800 B.C.-A.D. 200 in their Table 2). The overall impression is that “acorn was the most important plant food in the Southeast until Mississippian times when it was replaced in this position by corn” (Yarnell and Black 1985:97).

The Modern Plant and Its Distribution

Eastern North America is rich in oaks. Twenty-five species, not including hybrids, grow in Louisiana (Brown 1945), and 19 species are present in southern Illinois alone (Mohlenbrock 1986). Each species has its preferred set of environmental conditions, but most eastern hardwood forest communities contain several species.


Intersite comparison of the dietary importance of acorns is hindered by inconsistencies in analytical procedures. Some archaeobotanists continue pulling acorn shell fragments down to the 1.0 mm sieve, while others stop at 1.4 mm or even 2.0 mm. Any study that does not report acorn shell fragments smaller than 2.0 mm is inadequate for comparative purposes, since few carbonized pieces are larger than 2.0 mm.

All researchers recognize that acorn shell is under-represented archaeobotancially due to its fragility, but there is no accepted multiplication factor applied to yield a more equitable value for comparison to sturdier nutshell types such as hickory (Carya spp.). Lopinot (1982:743), who conducted valuable experimental carbonization tests, concludes that “1 g of carbonized acorn shell may represent the same quantity of meat as at least 12 to 56 g of hickory nut shell.”

Acorns are not considered food for people by most U.S. citizens today, and anthropologists may underestimate their importance in past diets due to the necessity of leaching out the bitter tannins. We have found, however, that acorn meats become tasty after only 20-30 minutes of boiling for the some sweeter (white oak subgenus) varieties, and one to three hours of boiling (with several changes of water) for the more bitter species (see also Petruso and Wickens 1984). Before dismissing acorns as an inferior food source, please try mixing acorn meal  with wheat and/or corn flour in your favorite banana bread or muffin recipe.


Brown, C. A.
    1945   Louisiana Trees and Shrubs. Louisiana Forestry Commission Bulletin No. 1. Baton

Hilliard, J. E.
    1986   ‘Selection and Use of Acorn Species by Late Prehistoric Ozark Inhabitants.’ In
            Contributions to Ozark Prehistory, edited by G. Sabo III, pp. 81-85. Arkansas
            Archaeological Survey Research Series No. 27. Fayetteville.

Lopinot, N. H.
    1982   ‘Plant Macroremains and Paleoethnobotanical Implications.’ In The Carrier Mills
            Archaeological Project: human Adaptation in the Saline Valley, Illinois, vol. 2, edited by
            R. W. Jeffries and B. M. Butler, pp. 573-860. Research Paper No. 33. Center for
            Archaeological Investigations, Southern Illinois University at Carbondale.

Mohlenbrock, R. H.
    1986   Guide to the Vascular Flora of Illinois. Southern Illinois University Press, Carbondale.

Olsen, D. F., Jr.
    1974   ‘Quercus L., Oak.’ In Seeds of Woody Plants in the United States, pp. 692-703.
            Forest Service, U.S.D.A. Agricultural Handbook No. 450, Washington D.C.

Petruso, K. M., and J. M. Wickens
    1984   ‘The Acorn in Aboriginal Subsistence in Eastern North America: A Report on
            Miscellaneous Experiments.’ In Experiments and Observations on Aboriginal Wild Plant
            Food Utilization in Eastern North America, edited by P. J. Munson, pp. 360-378.
            Prehistory Research Series, Vol. 6, No. 2. Indiana Historical Society, Indianapolis.

Steyermark, J. A.
    1963   Flora of Missouri. Iowa State University Press, Ames.

Yarnell, R. A., and M. J. Black
    1985   ‘Temporal Trends Indicated by a Survey of Archaic and Woodland Plant Food
            Remains from Southeastern North America.’ Southeastern Archaeology 4:93-106.

Written by:  Gayle Fritz