Bedrock Mortars

Numerous bedrock mortars are found at some sites and in some portions of San Diego County, but they are scarce or absent in many areas. Mortars differ in their immediate contexts and in their forms. From such variations, inferences have been proposed relating to chronology, ethnicity, and function.

Chronological issues concern the timing of the introduction of the mortar, the possibility that mortar use increased substantially during the Late Prehistoric period, and changes through time in mortar form. William J. Wallace (1955, 1962) suggested that the use of mortar and pestle was characteris­tic only of the Late Prehistoric period. Claude N. Warren (1964, 1968) argued that mortars made their first appearance in coastal San Diego around 3000 B.C. and were associated with the desert-derived Campbell tradition. For northern interior San Diego County, D. L. True (1958, 1980; True and Waugh 1981) excluded bedrock mortars from the early Pauma complex and included them in the San Luis Rey I and II complexes, also suggesting that a substantial increase in mortar use had occurred with San Luis Rey II. True and Georgie Waugh (1981:107) suggested that San Luis Rey I sites had mortars that were notably shallower than the mortars at San Luis Rey II sites, and that the former “would have been efficient only when used with a basket hopper”.

Ethnically, Luiseño territory in northern San Diego County has been observed to contain a higher ratio of bedrock mortars to bedrock basins and slicks than is found in Kumeyaay territory to the south. True (1966) suggested that the greater emphasis on manos and metates in Kumeyaay territory might reflect a stronger degree of cultural continuity from the Archaic period into the Late Prehistoric period, in contrast to a discontinuity associated with the “Shosho­nean intrusion” of Takic-speaking Luiseño in the north.

Functionally, mortars have most commonly been linked to the processing of acorns (e.g., Aschmann 1959:64). However, the ethnographic record indicates that a range of other plant food resources, small animals, and minerals for pigments were also processed in mortars (e.g., Barrows 1900; Bean 1972; Schroth 1996; Sparkman 1908). In the Jacumba/McCain Valley region, mortars were found to be dispersed among a wide range of different ecological settings, lending some support to the hypothesis of multiple functions for the features (Laylander et al. 2015). Michael A. Glassow (1997a) suggested that early mortar use in southern California was focused on root processing. Elsewhere in California, mortar depth has been proposed as a basis for distinguishing mortars used to process acorns from those used for seeds (McCarthy et al. 1985). Waugh (1986) noted differences in average mortar depths between upland and lowland Luiseño sites and suggested that this might relate to differences in the resources being processed. She observed that deeper mortars were more frequent at higher elevations.

Sites / Project	Ethnic Group (Heizer 1978)	Elevation (ft. AMSL)	Number of Mortars	Diameter (mm; mean and standard deviation)	Depth (mm; mean and standard deviation)	References
SDI-217	Luiseño	5240	221	165 ±56	93 ±73	Waugh 1986
SDI-308	Luiseño	2700	81	163 ±42	98 ±55	True et al. 1991
SDI-539	Luiseño	4650	46	204 ±56	153 ±73	Chace and Sutton 1978
SDI-649	Luiseño	400	15	171 ±28	55 ±39	Robbins-Wade 1991
SDI-674	Luiseño	120	36	214 ±48	103 ±65	Rosen 1984
SDI-682	Luiseño	300	114	166 ±35	113 ±38	True et al. 1991
SDI-5589	Luiseño	250	11	158 ±21	70 ±32	Fulmer 1985
SDI-7210A	Luiseño	1300	8	160 ±44	78 ±62	Chace and Hightower 1979
SDI-7313/7314	Luiseño	1050	8	157 ±28	93 ±29	Corum 1980a, 1980b
Frey Creek	Luiseño	820	32	150 ±35	55 ±39	True and Waugh 1981
SDI-799	Cupeño	2900	9	165 ±16	108 ±50	White et al. 1983
Cuyapaipe	Tipai	5400	28	194 ±–	119 ±81	Taylor and Carrico 1981
Kitchen Creek	Tipai	3150	15	153 ±29	—	May 1975
La Posta	Tipai	3800	33	188 ±24	117 ±54	Taylor et al. 1982

True, Rosemary Pankey, and Warren (1991) perceived some differentiation in mortar function both between sites in different settings and within individual sites. They proposed that intensive acorn processing, in contrast to multiple-resource processing, could be recognized on the basis of (a) a higher ratio of mortars to basins and slicks, (b) a higher ratio of mortar depth to diameter, and (c) a lower frequency of mortars superimposed upon or closely associated with other bedrock milling elements.

In the Jacumba/McCain Valley region, sites with mortars (n = 221) were found statistically to be located closer to water sources than were habitation sites or sites in general (Laylander et al. 2015). This may support the interpretation of mortars as having been used to process acorns, because the use of water to leach tannins from acorn mean is another essential element in its preparation.

Joan S. Schneider and Bonnie Bruce (2009) applied experimental crossover immunological electrophoresis (CIEP) analysis to groundstone features, including mortars, in Anza-Borrego Desert State Park, to test for residues of specific resources processed in the features. The test results supported the feasibility of the technique, although the limited sampling did not warrant any extensive conclusions on feature functions.

PROSPECTS

Future archaeological investigations may be able to clarify the range of functions for which bedrock mortars were used and any association between different functions and morphological differences. Studies may also clarify the chronology of mortar use and its association with specific ethnic groups. Evidence relevant to these issues will include dates for sites containing mortars, the geographical distribution and frequency of mortars, environmental contexts of mortars, variability in such mortar characteristics as diameter and depth, association of mortars with other types of milling features, and floral, faunal, and mineral residues on mortars and pestles.