Archaeological Investigations at Montaña de Oro State Park,

San Luis Obispo County, California

Site Structure & Chronology

The fog rolls in from the ocean at Montaña de Oro State Park

Soils and Stratigraphy

Cultural materials at CA-SLO-9 occurred within Still series soils, characterized by gravely sandy clay loams underlain by grayish brown gravely loam formed in colluvium and alluvium weathered from sedimentary rocks. The cultural layer is rather homogeneous, occupying the uppermost stratum (Stratum I), which was underlain by a sterile layer of decomposing bedrock (Stratum II). Soil characteristics were determined from soil samples taken from the field and an analyzed in the lab. Soil color was determined with Munsell Soil Color Charts (2000) and soil texture was determined through the “Sieve and Hydrometer” method, with the percentages of sand, silt, and clay determined through the “Bouyoucos” method.

West wall profile from CA-SLO-9.

Two variants of Stratum I were identified: I_A and I_B. Stratum I_A extends from the surface to a depth of between 5 and 20 cm. It ranged in color from very dark grayish brown to bluish black. The texture of stratum I_A ranged from a gravelly sandy loam (59.7% sand, 16.4% silt, 23.9% clay) to a gravely clay loam (46.6% sand, 16.7% silt, 36.7% clay) with moderate amounts of shell and a high density of cobbles and organic matter. Stratum I_B extends from the base of Stratum I_A to depths between 60 and 100 cm, with colors ranging from black to bluish black, and a texture that varies between gravely sandy clay loam (48.9% sand, 21.9% silt, 29.2% clay) and gravely sandy loam (64.8% sand, 20.1% silt, 15.1 clay). Stratum I_B is further characterized by a decrease in the density of organics and an increase in the density of shell relative to Stratum I_A. Stratum II consists of mostly decomposing shale bedrock and is marked by a sharp decrease in cultural material. The color of the soil within Stratum II ranged between very dark brown to light yellowish brown.

Soil pH values were determined from 10 samples. Soil solutions of a 2:1 water/soil ratio were measured and tested using a Fischer Scientific, Accumet Basic 15 pH meter. All strata were slightly basic: stratum I_A ranged from 7.79 to 8, stratum I_B ranged from 7.83 to 8.08, and stratum II ranged from 7.99 to 8.17. The basic character of the soil suggests that it provides a favorable environment for the preservation of faunal material.

Stratigraphic profile from CA-SLO-9, unit 4 and 8, west wall

Fire Affected Rock

Fire affected rock (FAR) was quantified only for Unit 10, the 1 x 1 m control unit. FAR density increased from 10 to 30 cm below the surface, but decreased from 30 to 90 cm. The data is compiled in the table below.

Chronological Methods

The chronology of CA-SLO-9 was established with seven radiocarbon dates that were compared with 21 temporally sensitive beads. Twenty-five obsidian hydration readings were also obtained, but owing to the difficulties in calibrating hydration dates, these were not considered seriously as independent evidence for the age of the occupations represented at CA-SLO-9. All of the samples used for radiocarbon dating were single marine shells. The shells were selected from a range of vertical and horizontal proveniences in attempts to define the full span of site occupation. Shells were submitted to Beta Analytic for analysis.. The radiocarbon ages were calibrated using the program CALIB 4.3, with a local upwelling correction factor of 290+35.

 

The seven radiocarbon dates suggest that the portion of the deposit investigated was occupied between cal. A.D. 920 and A.D. 1290 (one sigma probability), which is nearly synchronous with the Middle/Late Transition Period (ca. A.D. 1000 - A.D. 1250).  The intercepts obtained with CALIB 4.3 bracket this time period almost perfectly (A.D. 990 - A.D. 1240). This results are displayed in the table below.

Radiocarbon Analysis

Obsidian Hydration

Of 26 obsidian flakes sent for analysis, 25 returned measurable hydration rims. The majority of the specimens (n=16, 64%) came from the Coso source, with hydration rims that cluster between 3.7 – 4.4 microns. Eight specimens (32%) came from the Casa Diablo source with hydration readings clustering between 3.7 – 4.1 microns. A final specimen (n=1, 4%) came from the Napa source and has a hydration reading of 1.7 microns. The majority of obsidian hydration readings from the Casa Diablo and Coso sources fall within the Middle and Middle/Late Transition Periods (600 B.C. – A.D. 1250). Overall, the readings from Casa Diablo are more accurately inline with the radiocarbon results, while the hydration readings from the Coso source have several outliers in both temporal directions. The single reading from the Napa source is also considered an outlier in itself when compared with the readings from other sources and from the radiocarbon dates. These outliers may be explained by either (1) both older and more recent occupations or by (2) scavenged obsidian resulting in multiple hydration readings. Regardless, given that radiocarbon dating is a well attested method and that the radiocarbon dates overlap many of the hydration readings; it is safe to assume that the most intensive occupation occurred during this period of overlap.   

Coso and Casa Diablo obsidian hydration results from CA-SLO-9

Shell and Stone Beads

A total of 65 beads were recovered, the majority of which (n = 45) were Limpet Ring beads, which are not secure temporal markers, but which generally occur during the Middle Period in the Santa Barbara Channel. Normal saucers (G2) were the second most frequent bead (n=16) and three subtypes were identified: G2a (n = 2), G2b (n = 11), and G2d (n = 3), all of which date to the Middle Period and have been known to occur at Middle-Late Transition Period sites on the central coast. A single tiny saucer (G1) was also recovered and dates to the Middle-Late Transition. The last three beads date to the Late Period: two cupped beads (K1) and a single steatite disk.

Representative shell and stone beads from CA-SLO-9.

References Cited

Bennyhoff, J. A. and R. E. Hughes 1987 Shell Bead and Ornament Exchange Networks Between California and the Western Great Basin. American Museum of Natural History Anthropological Papers 64(2).

Ingram, B. L. and J. R. Southon 1996 Reservoir Ages in Pacific Coast Estuarine Waters. Radiocarbon 38:573–582.

Jones, T. L. 1993 Big Sur: A Keystone in Central California Culture History. Pacific Coast Archaeological Society Quarterly 2. Pacific Coast Archaeological Society, Costa Mesa.

King, C. D. 1982  The Evolution of Chumash Society: A Comparative Study of Artifacts Used in Social System Maintenance in the Santa Barbara Channel Region before A.D. 1804.  Ph.D. dissertation, University of California,

Davis, Stuiver, M. and Reimer, P. J. 2000 Calib 4.3 radiocarbon calibration program 2000. Quaternary Isotope Lab, University of Washington.