Past Environment

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 million 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.

Rising Sea Level
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 5,000 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 1st people settled by the marsh in Emeryville about 2,800 years ago, around 800 BC.

Geoarchaeologist Excavation & Findings
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 6 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.

Ever Changing Water Level
Water level in the bay has continued to rise at the rate of a centimeter or 2 a year into modern times. One evidence of this is the fact that the bases of many bay shore 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 where 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 bay shore probably would have gone virtually unnoticed by local inhabitants except that it also may have resulted in significant ecological changes in the bay shore. A shift occurs, sometime in the period between 2,000 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 bay shore, 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 bay shore 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.

Chemical Makeup of Shellfish
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 bay side 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 1 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.