Originally published in the Pacific Coast Archaeological Society Quarterly Vol. 27, No. 1, January 1991
CHRONOMETRIC DETERMINATIONS FOR THE
NORTHERN DEL REY HILLS,
LOS ANGELES COUNTY, CALIFORNIA
by
T.A. Freeman
INTRODUCTION
The Del Rey Hills of Los Angeles County comprise an expansive and irregular mesa-like landform which is bordered by the Los Angeles basin on the north and the Pacific Ocean on the west (figs. 1 and 2). The northernmost extent of the Del Rey Hills abruptly terminates at Centinela Creek, forming a series of bluffs (the Ballona Escarpment) which overlooks Santa Monica Bay.
Figure 1. General location of the northern Del Rey Hills plotted on a portion of the U.S.G.S. International Map of the Los Angeles Region.
Although most of the region is heavily developed, aerial photographs from the 1930's and 40's show countless meandering drainages and small ravines which emptied into the marshy playa below (fig. 3). The nearly vertical bluffs of the escarpment rising roughly 100 feet above the basin create a natural barrier which obstructs easy access to and from the sea. However, evidence suggests that in prehistoric times, the larger ravines provided natural passageways from the top of the escarpment to what is now Santa Monica Bay and the marine estuary environs of the playa lagoon (Van Horn 1987a:3ff.).
Figure 2. Approximate location of sites in the northern Del Rey Hills plotted on an image-enhanced satellite photograph of the Santa Monica Bay area.
An extensive series of archaeological investigations conducted by Archaeological Associates between 1983 and 1987 resulted in the identification of nine significant prehistoric deposits within this geographically and environmentally distinct region (Van Horn 1984; Van Horn and Murray 1985; Van Horn 1987b; Freeman, White, and Van Horn 1987). Seven of these sites are located on top of the escarpment overlooking the playa and two (LAn-62 and -211) are situated along the southern embankment of Centinela Creek at the base of the bluffs (fig. 2).
Figure 3. 1930 aerial photograph of the Ballona Escarpment along the northern edge of the Del Rey Hills, looking south. Playa Vista is in foreground.
Taken collectively, the excavations at these sites probably represent one of the largest and most unique series of archaeological investigations in coastal southern California. Most of the data recovery was accomplished using machine excavation and large-scale screening techniques which have since been widely published (Van Horn, Murray, and White 1986; Van Horn 1988; Freeman and Van Horn 1990:8f.). The use of these techniques in the Del Rey Hills yielded an abundant data base of artifactual and faunal material.
For example, excavations at LAn-63 (the Del Rey site) entailed the screening and processing of nearly 1,100 cubic meters of backdirt. A relatively small sample of this backdirt yielded 7,339 fish bones representing 63 different species, many of which had never before been identified at an archaeological site (Salls 1987:2). The combined volume of excavations at the nine Del Rey sites is estimated to have exceeded well over 4,000 cubic meters.
In addition to the extraordinary assemblages of cultural material, seven of the nine Del Rey sites (LAn-61A-C, -63, -64, and -206) yielded obsidian and/or marine shell samples which were analysed for chronometric purposes. These data represent a comprehensive and compelling suite of chronometric determinations for coastal Los Angeles County.
NOTE CONCERNING RADIOCARBON DETERMINATIONS
Twenty radiocarbon assays of marine shell valves were conducted in connection with the combined investigations along the Ballona Escarpment. Although this series of archaeological studies is pending publication, the results of the radiocarbon analyses have been documented elsewhere (Dillon and Boxt 1989; Breschini et al. 1988). However, there may be some controversy regarding these data. Two conflicting sets of dates have been published regarding these sites, and each could be interpreted as representing "absolute" readings.
The radiocarbon analyses for all of Archaeological Associates marine shell samples from the Del Rey Hills were conducted by Beta Analytic of Coral Gables, Florida. That laboratory provided two sets of probable ages for each sample ("C-14 Age Years B.P." and "C-13 Adjusted Age"). In their study, Archaeological Associates employed the C-13 adjusted age and an adjustment for Reservoir effect. Dillon and Boxt (1989) used the C-13/reservoir effect adjusted dates for comparative purposes in their publication on the Three Springs investigations in the Santa Monica Mountains. However, Breschini et al. (1988:11) published the C-14 Age B.P. data with no corrections.
In simple terms, the C-14 age assays are conventional uncorrected readings while the C-13 adjusted age is one which has been changed to reflect the isotopic fractionation of the sample species. The term "fractionation" refers to the tendency of species to preferentially absorb certain isotopes to the relative exclusion of others. Thus, the proportion of C-14 and C-13 atoms in the atmosphere may not have been reflected in the isotopic content of the living sample. Many researchers believe that marine shell is particularly susceptible to such alterations in radioactive isotopic absorption ratios (Tamers 1990:pers. comm.). Therefore, marine shell samples are believed to yield readings that are younger than the actual date; an error for which the C-13 adjustment compensates.
Conversely, reservoir effect occurs when sample valves are exposed to bicarbonate contaminants such as human-caused atmospheric radiation from atomic testing. These contaminants affect the chemistry of the valves in such a way that they yield exceedingly old dates. Robinson and Thompson (1981:48) have estimated that the dates of shell valves found along the southern California coast are approximately 690 +- 90 years older than their actual ages. Additional studies conducted by Stuiver et al. (1986) have produced a slightly revised reservoir effect rate of approximately 661 years.
Although some confusion may arise from the inconsistency, both suites of published radiocarbon data for the Del Rey Hills are "technically" correct. In fact, when adjustments are applied in tandem (for both C-13 and reservoir effect) a near cancellation of the adjustments occurs so that the difference becomes almost negligible (cf. "Uncorrected C-14 Age" and "Reservoir Effect Corrected" in Table 1). The average difference between conventional C-14 readings and combined isotopic and reservoir effect corrected readings for dates along the Ballona Escarpment is 268.3 years (younger than conventional readings).
Table 1. Radiocarbon determinations for sites in the northern Del Rey Hills.
When dealing with older assemblages, the discrepancy might have only slight significance. On the other hand, caution should be exercised when dealing with later time frames in which cultural sequencies are marked by the passage of only a few hundred years. It should also be noted that these effects on radiocarbon dates vary with regard to sample environment and it is possible for significant variation to occur in a relatively small geographic area (Breschini 1990:pers. comm.).
For the remainder of this paper, discussions concerning radiocarbon determinations will use uncorrected conventional C-14 readings.
NOTE CONCERNING OBSIDIAN HYDRATION DETERMINATIONS
In addition to the substantial radiocarbon data base, a combined total of 146 obsidian hydration samples were studied for hydration rim thicknesses. These specimens include those submitted to the UCLA Obsidian Hydration Laboratory by Dr. Brian Dillon and Mr. Vincent Lambert following their study of the region (cf. Meighan 1988:27 for a discussion). Additional samples were obtained during Archaeological Associates' investigation of the area. The UCLA Obsidian Hydration Laboratory provided test results on a total of 69 samples from the Del Rey Hills. Archaeological Associates conducted their own hydration analyses on the remaining samples using the methods described below:
"A wedge was removed from each obsidian flake or artifact by making two parallel cuts along the edge of the specimen using a watercooled, diamond impregnated copper alloy lapidary saw blade. The wedge was then obtained by breaking out the thin slice of obsidian between the two saw cuts. Initial grinding of the wedge was accomplished on a phenolic lapidary wheel impregnated with no. 600 artificial diamond grit. Use of this grit represents an innovation since most laboratories use corundum optical abrasives (e.g. Michels and Bebrich 1971:178). The polished surface of the wedge is then cemented to a slide using Canada balsam, the refraction index of which approximates obsidian. The wedge is then pressed firmly against the slide in order to insure that all wedges have approximately the same distance between the wedge and the slide surface...
"Final grinding is performed on the exposed side of the wedge after it has been mounted. Linscheid uses a phenolic wheel impregnated with no. 8000 diamond grit for this phase. The wedge must be ground to a thickness on the order of 30-50 microns so great care must be taken not to grind the wedge completely away. The finished slide is then placed on the stage of a microscope for hydration band measurement. We use a Nikon binocular metallurgical microscope fitted with a 100X oil immersion objective and a Vickerts image-splitting eyepiece. The latter device provides the measurements." (DiGregorio and Linscheid 1987:233f).
Frank Wood of the UCLA Obsidian Hydration Laboratory was consulted to confirm that Archaeological Associates' results would be commensurate with UCLA's findings (Van Horn 1987a:3). The results of a blind test showed that both laboratories derived similar readings from the same slides. Thus, all subsequent samples recovered from LAn-63 and -64 were processed by Archaeological Associates at their Sun City Laboratory. The combined results are provided in Table 2.
Table 2. Obsidian hydration determinations for sites in the nothern Del Rey Hills.
Dates are extrapolated using Meighan's 220 year/micron Coso source index.
Asterisks following laboratory numbers indicate specimens analysed by the
UCLA Obsidian Hydration Laboratory.
The absolute dates in this table were extrapolated from the hydration rim readings using Meighan's (1978) Coso obsidian index of 220 years/micron. No trace element analyses were conducted on the Del Rey Hills obsidian samples. However, the assumption that most, if not all, of the obsidian from the Del Rey Hills originated at the Coso source is not unreasonable considering that the vast majority of traceable specimens from Malibu, as well as those from other regional sites, are of Coso origin (Meighan 1988:27).
CHRONOLOGY
Based on the foregoing chronometric data, human occupation of the northern Del Rey Hills appears to extend from Millingstone Horizon times to the historic period. The data indicate that occupation may have begun as early as ca. 4800 B.C. with a possible terminus at about A.D. 1790. This broad occupational time frame is fragmented by perhaps several phases of complete or limited abandonment. The first and most notable of these may be represented by the nearly 2500 year lacuna bracketed between the 4800 B.C. and 2360 B.C. radiocarbon dates. The largest obsidian hydration rim readings are 20.0 and 18.4 microns (such thicknesses are rare). Applying Meighan's Coso index to these specimens suggests that they may have been knapped as early as ca. 2410 B.C. and 2058 B.C., respectively. These are very significant readings in that they could represent early obsidian trade activities.
The next significant gap in the chronological record appears between ca. 1200 B.C. and 2200 B.C. Although this may reflect an abandonment phase, it is also possible that assayable materials have yet to be discovered. On the other hand, human activity during the first millennium B.C. can be demonstrated with confidence. Drawing upon both radiocarbon and obsidian hydration anaylses yields nine dates for this time frame: B.C. 950, 720, 170, 122, 100, 34 (3), and 10.
"There is some reason to believe that the entire escarpment area, and perhaps the entire northern sector of the Del Rey Hills, underwent a period of abandonment or near abandonment following the close of the Millingstone Horizon. This statement is based on the fact that there is virtually no definitive evidence of human occupation during the second millennium B.C. Indeed, if all C-14 dates are corrected for reservoir effect, two dates of 700 and 480 B.C. are the earliest indications following Marymount's [LAn-61A] date of 2070 B.C. A few of the obsidian hydration readings are also in the first millennium B.C. range so that there is reason to believe that the escarpment began to experience renewed human use about this time" (Van Horn 1987c:266).
Use of Ballona Escarpment during the first thousand years of the Christian era is conclusively demonstrated by 125 obsidian hydration readings and 12 C-14 dates which fall within this period. Indeed, the bulk of the chronometric evidence seems to confirm that the northern Del Rey Hills area experienced its most intensive prehistoric use during the first thousand years A.D. More specifically, 90 obsidian hydration readings or 72% of the samples dating within this era cluster between A.D. 500 and A.D. 1000. Twenty-eight chronometric readings (twenty-six from obsidian and two from marine shell) extend this occupation into post-A.D. 1000 times. However, the majority of post-A.D. 1000 readings tend to group just prior to the 13th Century, with only one recorded for the historic period.
Figure 4. Graph expresses age-range distribution of obsidian hydration and radiocarbon dates for sites in the northern Del Rey Hills. Note: solid line indicates obsidian hydration dates calculated at 220 years/micron. Broken line represents uncorrected C-14 readings.
Depending on one's interpretation of the assemblages, it could be argued that certain time-sensitive artifacts, as well as those of somewhat ambiguous chronology, could span the hiatus in chronometric readings between the 14th and 18th centuries. However, as far as the author is aware, no assayable materials have yet been found to conclusively confirm this. One plausible explanation is that obsidian, which has provided the bulk of the chronometric evidence for the region, was being used less frequently after A.D. 1000. Meighan (1988:27) has proposed that following A.D. 1000, the obsidian trade from the Coso source was disrupted, thus encouraging a shift toward the use of fused shale in the manufacture of small artifacts.
"Unfortunately, what is being dated here is not the full span of occupation of the site [LAn-61B: Loyola], but rather the time of obsidian use and the period of obsidian trade from Coso. This period of obsidian use is seen to be about the first 1,000 years of the Christian era, based on numerous site samples in the Santa Monica Mountains and adjacent coastal areas" (Meighan 1988:27).
In this connection, however, it would seem that if obsidian was only a time-marker for trade, the presence or absence of the material would have little if any effect on C-14 readings. But this is not the case. Both C-14 and obsidian hydration readings are sparse for post-13th Century times. In fact, the distribution pattern of C-14 readings nearly mirrors that of obsidian (fig. 4). Perhaps the lack of obsidian, coupled with other factors, brought about significant economic pressures which led to a partial abandonment of the region.
One can see by the graph in Figure 4 that this abandonment or decreased use of the Ballona Escarpment occurred rather rapidly. In addition to obsidian trade disruption, it is hypothesized that environmental conditions of the playa, which at first were favorable to the initial settlement of the region, began to deteriorate after A.D. 1000, compelling the bluff's inhabitants to seek specific resources elsewhere. Evidence suggests that siltation of the lagoon disrupted estuary habitats, causing certain marine faunal resources to significantly diminish (DiGregorio 1987:299f; Van Horn 1987c:273f). Although this process may have taken a few centuries, the intense exploitation that the northern Del Rey Hills experienced during the first millennium A.D. eventually came to an end, finally to be replaced by a more casual and intermittent usage.
Even so, it seems that the area managed to generate some limited aboriginal resurgence during the 18th Century. Glass mission trade beads were recovered from LAn-61 and LAn-63. In addition, a single C-14 date of ca. 1790 was derived for LAn-61C. Thus, it is suspected that perhaps during the latter part of the late prehistoric era, or during proto-historic times, the Ballona Escarpment was re-occupied for a brief period before its final abandonment shortly following initial European contact.
REFERENCES
BRESCHINI, GARY S.
1990 Personal communication. Coyote Press. Salinas.
BRESCHINI, GARY S., TRUDY HAVERSAT, AND JON ERLANDSON
1988 California Radiocarbon Dates. 5th Edition. Coyote Press. Salinas
DiGREGORIO, LEE A.
1987 Marine Shell from the Del Rey Site. IN, Excavations at the Del Rey Site (LAn-63) and the Bluff Site (LAn-64), in the City of Los Angeles (D.M. Van Horn, ed.), pp. 220-231. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
DiGREGORIO, LEE A., AND JEFFREY V. LINSCHEID
1987 Chronometric Data. IN, Excavations at the Del Rey Site (LAn-63) and the Bluff Site (LAn-64), in the City of Los Angeles (D.M. Van Horn, ed.), pp. 232-238. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
DILLON, BRIAN D., AND MATTHEW A. BOXT
1989 Comparisons and Conclusions. IN, Archaeology of the Three Springs Valley California: A Study in Functional Cultural History (Brian D. Dillon and Matthew A. Boxt, eds.), pp. 138-169. Monograph 30. Institute of Archaeology, University of California, Los Angeles.
FREEMAN. T.A., AND DAVID M. VAN HORN
1990 Salvage Excavations at the Walker Ranch: A Portion of a Late Prehistoric and Historic Luiseno Village (CA-Riv-333). Pacific Coast Archaeological Society Quarterly, Vol 26, No 4, pp. 1-50.
FREEMAN, T.A., L.S. WHITE, AND D.M. VAN HORN
1987 Test Excavations at Two Sites Belonging to Howard Hughes Properties, Inc., in the City of Los Angeles. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
MEIGHAN, C.W.
1978 Obsidian Dating of the Malibu Site. IN, Obsidian Dates II: A Compendium of Obsidian Hydration Determinations Made at the UCLA Obsidian Hydration Laboratory (C.W. Meighan and P.I. Vanberhoeven, eds.). Monograph IV. Institute of Archaeology, University of California, Los Angeles.
1988 Obsidian Dates from the Loyola Site. IN, Obsidian Dates IV: A Compendium of Obsidian Hydration Determinations Made at the UCLA Obsidian Hydration Laboratory (Clement W. Meighan and Janet Scalise, eds.). Monograph XXIX. Institute of Archaeology, University of California, Los Angeles.
MICHELS, J.W., AND C.A. BEBRICH
1971 Obsidian Hydration Dating. IN, Dating Techniques for the Archaeologist. (Henry N. Michael and Elizabeth IC Ralph eds.), pp. 164-221. MIT Press. Cambridge.
ROBINSON, STEPHEN W., AND GAlL THOMPSON
1981 Radiocarbon Corrections for Marine Shell Dates with Application to Southern Pacific Northwest Coast Prehistory. Syesis, Vol. 14, pp. 45-57. The British Columbia Provincial Museurm
SALLS, ROY A.
1987 Fish Fauna from the Del Rey Site (LAn-63). IN, Excavations at the Del Rey Site (LAn-63) and the Bluff Site (LAn-64), in the City of Los Angeles (D.M. Van Horn, ed.), Appendix D, pp. 1-125. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
STUIVER, M., G.W. PEARSON, AND TOM BRAZIUNAS
1986 Radiocarbon Age Calibration of Marine Samples Back to 9000 CAL YR BP. Radiocarbon, Vol. 28, No. 2B, pp. 980-1021.
TAMERS, MURRAY
1990 Personal communication. Beta Analytic, Coral Gables.
VAN HORN, D.M.
1984 Mitigation of Impacts to Cultural Resources: Salvage Excavations at the Hughes Site (LAn-59). Unpublished report on file with Archaeological Associates, Ltd., Sun City.
VAN HORN, D.M.
1987a Introductory Material. IN, Excavations at the Del Rey Site (LAn-63) and the Bluff Site (LAn-64), in the City of Los Angeles (D.M. Van Horn, ed.), pp. 1-3. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
1987b Excavations at the Del Rey Site (LAn-63) and the Bluff Site (LAn-64), in the City of Los Angeles (D.M. Van Horn, Ed.). Unpublished report on file with Archaeological Associates, Ltd., Sun City.
1987c Concluding Commentary. IN, Excavations at the Del Rey Site (LAn-63) and the Bluff Site (LAn-64), in the City of Los Angeles (D.M. Van Horn, ed.), pp. 264-273. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
1988 Mechanized Archaeology. Wormwood Press. Calabasas.
VAN HORN, D.M., AND J.R. MURRAY (eds.)
1985 The Loyola Marymount Archaeological Project: Salvage Excavations at LAn-61A-C. Unpublished report on file with Archaeological Associates, Ltd., Sun City.
VAN HORN, D.M., J.R. MURRAY, AND R.S. WHITE
1986 Some Techniques for Mechanical Excavation in Salvage Archaeology. Journal of Field Archaeology, Vol. 13, pp. 239-244.
