Cortex on Lithic Debitage

Chronological signatures for lithic wastes would be particularly useful because of the ubiquity and relative abundance of debitage in most prehistoric assemblages. For many sites in San Diego County, lithic wastes are the only evidence available for interpretation or for making comparisons with other, more diverse assemblages.

Martin D. Rosen (1987a:80-89) proposed that a high frequency of cortex on debitage may be diagnostic of Archaic (La Jolla) assem­blages, as contrasted with Late Prehistoric assemblag­es. Rosen report­ed that an average of 30% of the debitage from a sample of Archaic assemblages con­tained cortex, while only 13% of the debitage from Late Prehistoric assemblages had cortex. Data from additional sites provide some support for Rosen’s general­ization, although considerable overlap in cortex percentages between the two periods is also apparent.

Site Main
Period		 Geologic
Setting		 Sample
Size		 Percent
with Cortex		 References
SDI-197 Archaic coastal 1,265 31 Rosen 1987a
SDI-1103 Archaic coastal 305 31 Laylander 1986a
SDI-4615 Archaic coastal 239 28 Laylander 1986a
SDI-6087 Archaic batholith 2,311 10 Cheever and Gallegos 1986
SDI-7197 Archaic coastal 178 21 Padon and Muranaka 1983
SDI-7197 Archaic coastal 4,788 11 Pigniolo and Gallegos 1990a
SDI-9243 Archaic batholith 17,065 13 Corum and White 1986*
SDI-9678 Archaic coastal 1,117 28 Rosen 1989
SDI-10,148 Archaic batholith 312 21 Carrico et al. 1991
SDI-11,570 Archaic volcanic 297 18 Pigniolo and Gallegos 1990b
SDM-W-110 Archaic coastal 1,602 13 Walker and Bull 1981*
SDM-W-1439 Archaic coastal 2,103 26 Hector 1984b
La Costa Archaic volcanic 904 30 Hector 1985b*
SDI-674 Late batholith 1,281 12 Rosen 1984*
SDI-4513 Late coastal 1,936 6 Eidsness et al. 1979*
SDI-4513 Late coastal 730 8 Rosen 1987b*
SDI-4513 Late coastal 3,831 25 Hector and Wade 1986*
SDI-4513 Late coastal 678 32 Gallegos et al. 1989
SDI-4609 Late coastal 6,198 19 Hector 1985a*
SDI-4763 Late volcanic 891 13 Rosen 1982*
SDI-5066 Late volcanic 886 18 Rosen 1982*
SDI-5383 Late coastal 6,867 14 Laylander 1989
SDI-5680 Late volcanic 18,496 14 Dominici 1985*
SDI-7306 Late volcanic 491 11 Pigniolo and Gallegos 1990b
SDI-8534 Late batholith 484 13 Laylander 1986b
SDI-8594A Late batholith 270 11 Carrico et al. 1991
SDI-9476 Late volcanic 10,005 19 Hector 1984a*
SDI-11,626 Late coastal 1,736 18 Pigniolo and Gallegos 1990a

* Cortex percentage as reported by Rosen 1987a.
Geologic Settings: coastal = sedimentary rocks of the coastal plain; volcanic = foothill/valley belt of Santiago Peak metavolcanics; batholith = valley/mountain belt of granitic and metamorphic rocks.

The presence or absence of cortex on lithic wastes may reflect the stage at which the wastes were produced within a lithic reduction sequence. One commonly applied classification, which distinguishes primary flakes with full dorsal cortex, secondary flakes with partial dorsal cortex, and tertiary flakes lacking cortex — implies such a sequence. The implication was made explicit in a nine-type lithic waste classifica­tion employed by Susan M. Hector (1984a), Rosen (1984), and others. In this scheme, small flakes and shatter that have cortex were assigned primarily to core preparation, initial reduction, and primary trimming activities, whereas small items that lack cortex were assigned to secondary trimming, shaping, maintenance, and modification activities.

Several possible explanations may be suggested for contrasts between different assemblages in the frequency of cortex:

  • The contrasts may reflect differences in the end-products intended by the knappers. Specifically, Archaic-period knapping may have been concerned to a large extent with manufacturing and maintaining large, roughly shaped unifacial tools, while Late Prehistoric knapping may have emphasized the fine working of small projectile points.
  • The contrasts may reflect differences in the lithic material sources that were used. Archaic knappers, living for the most part on coastal mesas and in coastal valleys, may have exploited primarily local cobble sources, with a high ratio of cortex to the volume of usable rock. Late Prehistoric knappers, based farther inland or making greater use of inland resources, may have made more extensive use of material quarried from outcrop sources, such as the Santiago Peak Vol­canics.
  • The contrasts may reflect differences in settlement organiza­tion. If Archaic peoples were near the foraging end of the for­aging/collecting continuum (Binford 1980; cf. Laylander 1997b), shifting their habitation bases to be near resources, they may have tended to bring relatively unworked lithic material to those bases for initial reduction. If Late Prehistoric peoples were more logistically organized collectors, they may have done most of the initial, cortex-removing reduction at quarries or special-function workshops.
  • In some instances, the contrasts may reflect differences in archaeological recovery techniques, such as the inclusion or exclusion of surface collections or the size of the screen mesh used. Within particular assemblages, smaller pieces of debitage have been found consistently to have lower frequencies of cortex.

PROSPECTS

Future archaeological investigations may be able to determine whether the frequency of cortex on lithic debitage is a usable index of site chronology, or whether differences in this variable are explained by other factors.