Still, archæological excavation has brought to light something. It has shown that the ancient implements found in shellmounds and village sites in Southern California, those from the shores of San Francisco Bay in Central California, and those along the coast of Northwestern California, are distinct. Certain peculiar types of artifacts are found in each of these regions, are found only there, and agree closely with objects used by the modern tribes of the same districts. For instance, prehistoric village and burial sites in Northwestern California contain long blades of flaked obsidian like those used until a few years ago by the Yurok and Hupa. Sites in Southern California have brought to light soapstone bowls or “ollas” such as the Spaniards a century ago found the Gabrielino and Luiseño employing in cooking and in jimsonweed administration. Both these classes of objects are wanting from the San Francisco Bay shellmounds and among the recent Central Californian tribes.
It may thus be inferred (1) that none of the four local cultures was ever spread much more widely than at present; (2) that each of them originated mainly on the spot; and (3) that because many of the prehistoric finds lie at some depth, the local cultures are of respectable antiquity—evidently at least a thousand years old, probably more. This fairly confirms the estimate that the differentiation of the local cultures of the Third Period commenced not later than about 500 A.D.
167. Age of the Shellmounds
Archæology also yields certain indications as to the total lapse of time during the four periods. The deposits themselves contribute the evidence. Some of the shellmounds that line the ramifying shores of San Francisco Bay to the number of over four hundred have been carefully examined. These mounds are refuse accumulations. They were not built up with design, but grew gradually as people lived on them year after year, because much of the food of their inhabitants was molluscs—chiefly clams, oysters, and mussels—whose shells were thrown outdoors or trodden under foot. Some of the sites were camped on only transiently, and the layers of refuse never grew more than a few inches in thickness. Other spots were evidently inhabited for many centuries, since the masses of shell now run more than thirty feet deep and hundreds of feet long. The higher such a mound grew, the better it drained off. One side of it would afford shelter from the prevailing winds. The more regularly it came to be lived on, the more often would the inhabitants bring their daily catch home, and, without knowing it, thus help to raise and improve the site still further.
Some of these shellmounds are now situated high and dry, at some distance above tide water. Others lie on the very edge of the bay, and several of these, when shafts were sunk into them, proved to extend some distance below mean sea level. The base of a large deposit known as the Ellis Landing mound, near Richmond, is eighteen feet below high tide level; of one on Brooks island near by, seventeen feet. The conclusion is that the sites have sunk at least seventeen or eighteen feet since they began to be inhabited. The only alternative explanation, that the first settlers put their houses on piles over the water, is opposed by several facts. The shells and ashes and soil of the Ellis Landing mound are stratified as they would be deposited on land, not as they would arrange in water. There are no layers of mud, remains of inedible marine animals, or ripple marks. There is no record of any recent Californian tribe living in pile dwellings; the shore from which the mound rises is unfavorably situated for such structures, being open and exposed to storms. Suitable timber for piles grows only at some distance. One is therefore perforce driven to the conclusion that this mound accumulated on a sinking shore, but that the growth of the deposit was more rapid than the rise of the sea, so that the site always remained habitable.
How long a time would be required for a coast to subside eighteen feet is a question for geologists, but their reply remains indefinite. A single earthquake might cause a sudden subsidence of several feet, or again the change might progress at the rate of a foot or only an inch a century. All that geologists are willing to state is that the probability is high of the subsidence having been a rather long time taking place.
The archæologists have tried to compute the age of Ellis Landing mound in another way. When it was first examined there were near its top about fifteen shallow depressions. These appear to be the remains of the pits over which the Indians were wont to build their dwellings. A native household averages about 7 inmates. One may thus estimate a population of about 100 souls. Numerous quadruped bones in the mound prove that these people hunted; net sinkers, that they fished; mortars and pestles, that they consumed acorns and other seeds. Accordingly, only part of their subsistence, and probably the minor part, was derived from molluscs. Fifty mussels a day for man, woman, and child seem a fair estimate of what their shellfish food is likely to have aggregated. This would mean that the shells of 5,000 mussels would accumulate on the site daily. Laboratory experiments prove that 5,000 such shells, with the addition of the same percentage of ash and soil as occurs in the mound, all crushed down to the same consistency of compactness as the body of the mound exhibits, occupy a volume of a cubic foot. This being the daily increment, the growth of the mound would be in the neighborhood of 365 feet per year. Now the deposit contains roughly a million and a quarter cubic feet. Dividing this figure by 365, one obtains about 3,500 as the presumable number of years required to accumulate the mound.
This result may not be accepted too literally. It is the result of a calculation with several factors, each of which is only tentative. Had the population been 200 instead of 100, the deposit would, with the other terms of the computation remaining the same, have built up twice as fast, and the 3,500 years would have to be cut in half. On the other hand, it has been assumed that occupation of the site was continuous through the year. Yet all that is known of the habits of the Indians makes it probable that the mound inhabitants were accustomed to go up into the hills and camp about half the time. Allowance for this factor would double the 3,500 years. All that is maintained for the computed age is that it represents a conscientious and conservative endeavor to draw a conclusion from all available sources of knowledge, and that it seems to hit as near the truth as a calculation of this sort can.
One verification has been attempted. Samples of mound material, taken randomly from different parts, indicate that 14 per cent of its weight, or about 7,000 tons, are ashes. If the mound is 3,500 years old, the ashes were deposited at the rate of two tons a year, or about eleven pounds daily. Experiments with the woods growing in the neighborhood have shown that they yield less than one per cent of ash. The eleven daily pounds must therefore have come from 1,200 pounds of wood. On the assumption, as before, that the population averaged fifteen families, the one-fifteenth share of each household would be eighty pounds daily. This is a pretty good load of firewood for a woman to carry on her back, and with the Indians’ habit of nursing their fires economically, especially along a timberless shore, eighty pounds seems a liberal allowance to satisfy all their requirements for heating and cooking. If they managed to get along on less than eighty pounds per hut, the mound age would be correspondingly greater.
This check calculation thus verifies the former estimate rather reasonably. It does not seem rash to set down three to four thousand years as the indicated age of the mound.