Clam shell disk beads, a marker of the Late Prehistoric Period in Central California

Between the 1939 and 1968 investigations, archaeological research interests had changed somewhat. While the earlier group had excavated large trenches looking for human burials, the later archaeologists dug several small, square pits or "units," spread randomly across the site. They wanted to document the full range of activities that took place at the site over time, such as the making of tools, the preparation of food, and any other events that would help them reconstruct the daily life of the site's prehistoric inhabitants.

Archaeological field methods also had changed since the Brazil Mound was first studied. The methods used at the site in 1968 are still followed in the 1990s: digging in 10-centimeter levels⁵, screening all excavated soil through fine mesh, collecting all materials that might provide clues about the life of the people - charcoal from their fires (for radiocarbon dating), chipping waste from the making of stone tools, bones and shells from the animals and shellfish they ate. In their slow, careful digging, the archaeologists also found the remains of several fire hearths (some containing bone, shell, and artifacts), as well as possible post holes defining the location of houses or other structures, and a scatter of clamshell disc beads.

The clamshell bead scatter was a curious find. The archaeologists from U.C. Davis knew from the style and size of the beads that they were much later than the other artifacts from the site, and they were at a loss to explain their presence in the midden. Fortunately, Francis Brazil was able to provide an explanation: the beads had been collected years earlier from another site in the delta, CA-SAC-56, and kept in the house in a glass jar. The jar had apparently broken in the heat of the 1966 fire, and the beads had mixed with artifacts from the mound!

If not for the explanation provided by the land owner, the archaeologists probably would have assumed that Native Californians had lived at the Brazil Mound later in time than they actually had. This is only one example of the many ways the archaeological record can be altered, making the past more difficult for us to reconstruct. When artifacts and other items are removed from a site by hobbyists, treasure hunters, or vandals ("pot-hunters"), or during

⁵ - Archaeological deposits are dug in levels to keep the lower (and presumably earlier) materials separate from the upper (later) ones.