Coastal Morphology

The San Diego coastline exhibits a complex pattern of sea cliffs, sandy beaches, bays, lagoons, and sloughs. Within the timescale of the Holocene, these features have not been permanent. Changes in the physical environment imply changes in the associated nearshore fauna, which in turn must have required changes in human adaptations to those resources.

At the time of the last glacial maximum, about 20,000 B.C., sea level was approximately 120 meters below its present level. Subsequent levels have been estimated as -100 meters in 13,000 B.C., -40 meters in 8000 B.C., and at its present level by 3000 B.C. (Masters and Aiello 2007; Peltier 2002). Littoral sites occupied by Pleistocene human inhabitants are likely to have subsequently been drowned or destroyed. Underwater archaeology has identified some offshore artifacts, apparently in situ (e.g. Masters 1983), but much remains to be determined concerning the contexts and chronology of such finds.

One effect of the rapid rise in sea level at the end of the Pleistocene may have been to create substantially more rocky shoreline than exists at present. George Shumway, Carl L. Hubbs, and James R. Moriarty (1961) argued that the predominance of mussel (Mytilus spp.) shell in the lower levels of the Scripps Estate Site (SDI-525) was indicative of such conditions. The gradual accumulation of a sand buffer, as sea level stabilized, may account for the decrease of mussel in later deposits. It has been suggested that the gradual growth of sandy beaches may also account for the late emergence of focused exploitation of Pismo clam (Tivela stultorum) and bean clam (Donax gouldii) (Laylander and Saunders 1993; Masters 2003, 2005; Masters and Aiello 2007).

During the Pleistocene, rivers and creeks were able to carve valleys more deeply than at present into the relatively soft sedimentary rocks of the coastal plains. When the sea level rose, the lower portions of the river valleys were drowned, creating estuaries. Patricia M. Masters’ (1988) study of the San Diego Bight is a detailed examination of a local late Pleistocene and early Holocene marine transgression. According to Masters, San Diego Bay took shape as late as about 5000 B.C., prior to which the Point Loma drainage, Chollas Creek, Sweetwater River, and Otay River all reached the open coast.

As sea level became stable, the estuaries functioned as traps for the silt and sand that were fed into them by the rivers. Along the open coasts, sands and gravels were moved by longshore currents to form spits that intermittently or permanently blocked estuary mouths. The estuaries ultimately tended to disappear, except insofar as strong river currents or tidal flushing removed the sediments and unblocked their mouths. Masters (1988) suggested that large rivers would rapidly fill their embayments, whereas the bays associated with smaller streams would be longer-lasting.

The late Holocene transformation of Batiquitos Lagoon has been the subject of detailed studies. Claude N. Warren and his collaborators argued that the Batiquitos area was largely abandoned by prehistoric peoples after about 1000-500 B.C. because there was no longer an adequate supply of shellfish to be exploited (Warren 1964; Warren and Pavesic 1963; Warren et al. 1961). Carl L. Hubbs and others (Hubbs et al. 1962:222; Shumway et al. 1961) cited three radiocarbon dates on Chione undatella shells from middens near Batiquitos as indicating that the lagoon was open around A.D. 1000. Jacqueline Miller (1966) found evidence that Batiquitos Lagoon had been closed by around 1500 B.C. but had reopened between A.D. 500 and 1200. Dennis R. Gallegos (1985, 1987b) saw radiocarbon dates from sites around Batiquitos as evidence for a scenario similar to Miller’s: that the lagoon had closed around 1000 B.C. and then reopened during the period between about A.D. 370 and 1445.

The lower San Luis Rey River has also been studied in some detail. Antony R. Orme (1992; Vanderpot et al. 1993) suggested that the San Luis Rey estuary may have initially reached as far as 4 kilometers inland, whereas Masters (1992) proposed that it had extended as far as 11 kilometers. According to Rein Vanderpot, Jeffrey H. Altschul, and Donn R. Grenda (1993), a narrow constriction of the San Luis Rey valley only about 2 kilometers inland from the present coastline would probably have made any saltwater or brackish estuary farther east than this point very short-lived. Pollen and macrofloral evidence from the Whelan Lake Site (SDI-6010), dated to about 5000 B.C., was cited as indicating the presence of a freshwater setting at that period. A shift in the predominant shellfish resource used at the Whelan Lake Site from venus clam (Chione spp.) and scallop (Argopecten sp.) during the Archaic Period to bean clam (Donax gouldii) during the Late Prehistoric Period was attributed to siltation of the lower estuary (Vanderpot et al. 1993).

Kevin O. Pope (2003) used four sediment cores from near the mouth of Las Flores Creek to reconstruct the eastward shift of the coastline by about 3 kilometers between 9,000 and 4,000 years ago. His observations included the presence of a bay, slough, and lagoon at the present river’s mouth about 6000 B.C. and their disappearance after 4000 B.C.

Using archaeological evidence from five sites around Mission Bay, Alette van den Hazenkamp argued that changes during the Holocene at this location had been “far less evident or not visible at all compared to San Diego Bay and other nearby lagoons” (van den Hazenkamp 2011:1).

A number of prehistoric cultural responses to the changing configuration of the San Diego coastline have been suggested:

• An early Holocene shift from an adaptive strategy based on land mammal hunting (the San Dieguito Complex) to one based on littoral collecting (the La Jolla Complex) may have been a response to the emerging availability of rich estuary shellfish resources. A factor to be considered in evaluating this view is the possible burial or destruction of early sites that may have been located either to the west of the present coastline or in aggrading alluvial valleys.
• The preponderance of particular shellfish genera (Mytilus, Ostrea, Argopecten, Chione, Tivela, and Donax) in prehistoric diets may have shifted in response to physical changes in estuarine environments. Mussel, oyster, and gastropods, living on rocky substrates, may have been most abundant in the immediate aftermath of a rapid rise in sea level. Scallop exploitation may have flourished at bays and lagoons with sandy substrates, but may have been gradually replaced by venus clam as the lagoons became more shallow and unstable. Finally, there may have been a shift to Pismo and bean clam exploitation with the closure of the lagoons and the emergence of sandy open beaches. On the other hand, at Newport Bay in Orange County, Elizabeth M. Wojdak (1993) argued that a change between late Archaic and Late Prehistoric Periods from a predominance of scallop to a predominance of venus clam in shellfish exploitation was a cyclical rather than a cumulative phenomenon, reflecting a shift from warmer and drier to cooler and moister climatic conditions and a concomitant increase in the amount of sediment and the instability of bottom conditions in the estuary.
• A late Archaic or Late Prehistoric shift in settlement away from the coast in the central and northern portions of San Diego County may have occurred as a response to the deterioration of the estuaries as resource areas (Warren 1964; Warren et al. 1961). Some of the estuaries, such as Batiquitos Lagoon, may have had more complex histories involving both closure and subsequent reopening. Brian F. Byrd and Seetha N. Reddy (2002; Byrd 1998) have questioned whether any such general shift occurred. 

PROSPECTS

Future archaeological and geological investigations may be able to define the timing, character, and local variability of coastal changes during the late Pleistocene and Holocene, and the prehistoric cultural responses to them. Archaeological evidence relevant to these issues will include the characterization of natural sediments in dated column samples, confirmation of the presence of submerged archaeological deposits, changing patterns in the exploitation of coastal resources as attested in archaeological middens, and evidence for changing site functions and settlement patterns.