Cupules are small concavities, ground or pecked into boulder or bedrock surfaces and typically ranging from 2 to 8 centimeters in diameter. In the Great Basin, cupules have been termed the "pit-and-groove petroglyph style" (Heizer and Baumhoff 1962), but grooves are almost never associated with the features in San Diego County. Occurrences of single cupules and of arrays of them running into the hundreds have been reported. The features often occur on vertical as well as on horizontal surfaces and have generally been interpreted as a form of rock art rather than as items shaped by or for functional use. Distinguishing cupules from concavities produced by natural weathering or from small or incipient mortars may be problematical in some instances. For southern California, focused studies of cupules have been carried out by Rick Minor (1975), Ken Hedges (1973, 1980), and Susan Hector (2009).
The chronological placement of cupules is uncertain. In the Great Basin, the proposed time ranges are generally early: 7000 to 5000 B.C., according to Jay C. von Werlhof (1965:120); 5000 to 3000 B.C., according to Robert F. Heizer and Martin A. Baumhoff (1962:234); and 5000 B.C. to 500 B.C., according to Polly Schaafsma (1986). In Hedges' (1973:21) review of records from San Diego and Riverside counties, he reported that all of the cupule sites "were associated with late archaeological sites, indicating placement of cupule petroglyphs within the last 500 years." Minor (1975) reached a similar conclusion. D. L. True, C. W. Meighan, and Harvey Crew (1974) suggested that cupules at Molpa (SDI-305) in Pauma Valley belonged to the San Luis Rey I rather than the San Luis Rey II component, a conclusion based in part on weathering and lichen growth. True and Baumhoff (1981) disputed the suggestion that cupule use continued as late as the Late Prehistoric period, at least within Luiseño territory. The assignment of cupules to an earlier date by True and Baumhoff was based in part on a correspondence between the areas in which cupules occur and areas of Hokan speech, and in part on the scarcity or absence of reliable ethnographic testimony concerning cupules.
Geographically, cupules seem to occur most commonly in the mountains and in the western foothills of San Diego County. This may in part reflect the availability of large boulders suitable as cupule surfaces in those areas.
Motives for the manufacture and use of cupules have been suggested on the basis of ethnographic information from various parts of California:
-- Robert F. Heizer (1953) noted that some northern California groups, including the Shasta, made cupules as "rain rocks" to control weather.
-- E. Breck Parkman (1992, 1993) hypothesized that large cupules on horizontal surfaces in southern California sites caught rainwater that was used for ritual or other purposes.
-- The Pomo of northern California termed cupules "baby rocks." The features were pecked by women who wanted to become pregnant (Aginsky 1939; Barrett 1952; Loeb 1926).
-- There have been several suggestions that the Luiseño produced cupules as part of their puberty rites, either for boys or for girls or perhaps for both (Chace 1964; DuBois 1908; Hedges 1976:17). True and Baumhoff (1981) disputed this interpretation of the ethnographic record.
-- Some southern California peoples are said to have had "death stones," on which a cupule was formed when someone in the community died (Ewing 1948).
-- The Luiseño are reported to have made hollows in rocks as territorial ownership markers (DuBois 1908:158).
-- The Cahuilla reportedly used cupules as trail markers (Patencio 1943:98).
Additional inferences concerning the functions of cupules have been suggested on the basis of various archaeologically observed characteristics or associations of the features:
-- Heizer and Baumhoff (1962) and von Werlhof (1965) reported that cupules in the Great Basin were associated with game trails rather than with habitation sites, and that the features therefore were probably relatable to hunting magic. Few southern California cupules are associated with trails (Minor 1975), although it is also true that few trails have been preserved or identified in the general region containing the features. Twenty-six sites with cupules in the Jacumba/McCain Valley region were not found to be associated with trails and only weakly represented at specialized resource processing areas; the majority (n = 21) were habitation sites (Laylander et a. 2015).
-- Minor (1975) noted that an association of petroglyphs with springs had been suggested, but that few southern California cupules are associated with springs. A study of site records for the Jacumba/McCain Valley region found that sites with cupules were located somewhat closer to springs on average than were habitation sites or sites in general (Laylander et al. 2015).
-- With regard to cupules that are situated on vertical surfaces, there seems to be no particular bias in the directional orientation of such surfaces (Minor 1975; von Werlhof 1965). However, Hedges (1980) argued for the importance of continuing to watch for potential patterning in this variable.
-- Minor (1975) noted that cupules in southern California rarely form any discernible patterns in their arrangement on a particular outcrop. However, Hedges (1980) suggested that patterns may be present but not recognized by most observers; he reported that cupules at a site within Kumeyaay territory in northern Baja California formed a pattern closely matching the major stars of the constellation Cassiopeia.
-- Hedges (1980) noted that some observers have interpreted cupules functionally as utilitarian features for cracking nuts or as paint mortars. The occurrence of the features in abundance at a few sites but their absent from most habitation sites seemed to tell against the suggestion. According to Minor (1975), cupules in southern California occur fairly frequently in association with pictographs and with other forms of petroglyphs; this would support the interpretation of cupules as rock art. However, in the Jacumba/McCain area, only two out of 45 sites with petroglyphs also contained cupules (Laylander et al. 2015).
-- True, Meighan, and Crew (1974) reported two varieties of cupules or cupule-like features at the Molpa Site. One variety, with depressions about 2-5 centimeters in diameter, corresponded to conventional cupules. The second variety was observed on a single "rainrock" with hundreds of small depressions about 1 centimeter in diameter. The latter feature was considered to be unique in this region; one similar feature was known on Santa Catalina Island, and "rainrocks" were said to be not uncommon in northern California.
-- True (1993) argued that at least some cupules on horizontal surfaces functioned as acorn hulling pits. This was considered particularly likely when they were closely associated with bedrock mortars. In the Jacumba/McCain Valley area, of 188 specialized resource processing sites with bedrock milling features, only two also contained cupules (Laylander et al. 2015).
-- Joan S. Schneider and Bonnie Bruce (2009) applied crossover immunological electrophoresis (CIEP) analysis to cupules as well as other ground stone features in Anza-Borrego Desert State Park. Three cupules returned positive results: one for cat; one for Capparidaeceas (bladderpod, etc.) and mesquite; and one for rat. These preliminary results seem to suggest that at least some cupules may have had utilitarian rather than ritual functions.
-- Mark Q. Sutton, David W. Robinson, Gale Grasse-Sprague, and Jack Sprague (2011:91) suggested that "some manufactured cupules could have been used as 'containers' to hold a variety of physical or supernatural substances." The hypothesis was based on materials found within cupules at a site in Kern County.
Future archaeological research may be able to clarify the chronological placement of cupules; their association with habitation and other site types, water sources, and probable prehistoric travel routes; patterning in their size, orientation, and arrangement; and the circumstances of their creation and use as indicated by wear patterns or use residues.