The San Francisco Bay Area has experienced dramatic
environmental changes over the last 10,000 years. These changes were the result of natural climatic fluctuations and,
more recently, human influences. When looking west across San Francisco Bay at the
Golden Gate Bridge and beyond, it is hard to imagine that 10,000 years ago the San
Francisco Bay probably did not exist. Instead, the basin where the bay now lies was a
large valley drained by rivers that flowed to a coastline 30 miles west in the vicinity of the
Farallone Islands. Sea level was hundreds of feet lower than it is
today. The coastline of
that time is now deep under water.
During the past 2,000,000 years the earth
has seen long series
of Ice Ages, periods during which huge masses of ice formed on the continents. These ice masses in effect "locked
up" ocean waters, with the result that during these epochs there was substantially less
water than today in oceans throughout the world. This drop in sea level not only exposed
wide areas of coastline, but also affected the topography of inland areas. However,
around 10,000 years ago, a natural global warming trend began. As the continental
glaciers melted, sea levels began to rise rather rapidly. By about 8,500 years ago
the rising sea began to flood in through the Golden Gate flood to form the San Francisco
Bay. Although people lived in California and probably in the Bay Area by that time, no
archaeological sites have been found on the shores of the San Francisco Bay to evidence
this occupation. Likely the rising bay waters inundated any such sites.
By about 5000 years ago the rate of sea level rise
slowed, and extensive marshes began to form around the shores of the bay. These marshes
provided a rich habitat
for plants and animals, and soon began to attract human settlement. By
3,500 years ago,
people had begun to settle on the bay shores in significant numbers. The first people
settled by the marsh in Emeryville about 2800 years ago, around 800 BC.
A number of specialists worked side-by-side with the
archaeologists on the Bay Street Project to gather and interpret evidence of past environments. During
archaeological excavations at the site, a geoarchaeologist excavated a series of trenches
about 10 feet deep in numerous areas around the site. These excavations exposed cross-sections of the
mound and underlying and adjacent soils, which provided information about the geologic
setting in which the site was formed. Several trenches exposed evidence of former creek
channels. Findings here suggest that the earliest settlement of the site was on small
natural rises on sediments deposited along the banks of Temescal Creek. Later, after the
shellmound deposit had reached a height of six feet, the creek meandered across the
mound again. It removed some of the deposit in some areas, and
buried the deposit under silt in other areas. Eventually the growing mound
came to act as a berm on the edge of the creek, and probably regulated its course
somewhat.
Water level in the bay has continued to rise at the rate of a
centimeter or two a year into modern times. One evidence of this is the fact that the bases of many bayshore mounds,
including Emeryville, are inundated. However, inundation of the base of the mound--which surely was on dry land when it
was formed--may also be evidence of significant tectonic subsidence (dropping of the land which the site lies), which
has occurred in many places in the Bay Area. This may have happened gradually, or through a series of
sudden events (like earthquakes), or even through a single major
earthquake. However, even if
this subsidence did occur rather suddenly, by this time the "superstructure" of the mound
had built up to the point that there was plenty of dry area above the marsh level for
occupation.
In fact, the (hypothesized) subsidence of the bayshore
probably would have gone virtually unnoticed by local inhabitants except that it also may have resulted in significant
ecological changes in the bayshore. A shift occurs, sometime in the period between 2000
years ago and 800 years ago, in the shellfish remains predominant in many shellmounds
along the bay. In general, early in the human occupation of the
bayshore, mussels and oysters were predominate at many sites.
These shellfish species thrive on
rocky shores and gravel beds. Later in time, while mussels and oysters still are present, clams, which are mud dwelling
species, become the predominant shellfish species at some sites. The hypothesized subsidence of the
bayshore could have changed the gradient of creeks entering the bay, such
that the waters slowed and began to deposit higher amounts of silt closer to shore. If this occurred, it
could have resulted in the accelerated growth of shoreline marshes and the burial
by alluvium of former rocky and gravely shores. This would have changed shellfish habitat,
and might have resulted in a shift from rock dwelling to mud dwelling species in
archaeological assemblage.
Another study of environmental change at Emeryville
involved an examination of the chemical makeup of a mussel shell from the mound, in comparison with a modern mussel
shell. Shellfish accrete new shell layers seasonally, and the chemical composition of these
layers varies with the salinity of the water in which they grow. During rainy seasons and
times of rapid runoff in bayside creeks, the bay water near the shore is less salty.
Conversely, the water becomes more saline in dry seasons and periods of drought.
Although only one sample from Emeryville was subjected to this chemical analysis,
results of the analysis suggest that this technique may be useful in documenting
prehistoric climatic fluctuations around the bay. This would be useful in understanding
other changes in the plants and animals evident in the site, as well as
archaeological periods of more or
less intensive occupation by prehistoric people.
For more discussion of plants and animals in the prehistoric environment, see
Use of Plants and Animals.
