[12] The shellmounds in the vicinity of the bay differ considerably in shape and size. The majority appear as extended plateaus 10 to 12 feet in height, others appear as slight undulations of the ground about five feet in height. The truncated conical form is found more rarely; the mound at Ellis Landing near Point Richmond approaches it somewhat in its proportions. Many of these mounds cover acres of ground, e.g., the mounds of Alameda, of Sausalito, of Sierra Point, of West Berkeley (in its older form, now much changed). In tropical regions many shellmounds are said to reach a height of 100 feet or more; this is known with certainty of some in Brazil (cf. Nadaillac, l. c., p. 54), and also of two near the dried-up mouth of the Ica river in Peru. Shellmounds as a rule are much smaller. On the Atlantic coast near Smyrna a shellmound is said to be thirty feet high (Short, l. c., p. 107), but the majority of these mounds are less than four feet high (cf. Wyman, Amer. Naturalist, 1868, I, p. 56 ff., and Abbott, l. c., p. 440), while many of them extend over areas of more than two or three acres. A shellmound near the mouth of the Altamaha river in Florida is estimated as having a size of over 80,000 cubic yards (Smithson. Rep., 1866, p. 358). The shellmounds of Denmark are only from 3 feet to 10 feet high, although more than a thousand feet long (Ranke, Der Mensch, II, p. 552). Southern California shellmounds generally are from 4 feet to 5 feet high (P. Schumacher, Bull., l. c., p. 38; and Smithson. Rep., 1874, p. 337, etc.). The same is the case with those mounds on the Aleutian Islands explored by W. J. Dall. In Oregon there are some of at least 8 feet in height (cf. Schumacher, l. c., p. 29).
[13] Shellmounds in the bay region are mostly in localities where there is fresh water, a creek or a spring, generally the former. W. H. Dall (Contributions, p. 34) observes that for the formation of shellmounds on the Aleutian Islands two conditions are necessary, as a rule: running water or a spring, and a site suitable for boat landing; one or the other of these conditions lacking, no shellmounds are to be found. In Oregon the shellmounds are generally to be found near a creek (cf. Schumacher, l. c., p. 28). The same rule probably governs the shellmounds of the East. D. G. Brinton found shellmounds in Florida generally near running water (Smithson. Rep., 1866, p. 356), but he supposes as the cause of this the greater abundance of shells near the mouths of rivers, while it is certain that the presence of drinking water was the main attraction.
[14] Parts of the eastern coast of the United States are sinking. Several shellmounds on the Jersey coast are being washed away at present (cf. Abbott, l. c., p. 448 ff.). The same may be observed with the shellmounds near Ellis Landing on the Bay of San Francisco.
[15] Near the mouth of the valley of San Rafael a small hill rises from the bay, the isolation of which from the mainland may be explained in this way.
[16] Between the shellmounds of Emeryville and West Berkeley the shore for a long stretch forms a steep bank up to twelve feet high, and broken down by the water of the bay. Possibly the coast at this point formed a promontory on the two sides of which these shellmounds were originally founded, as in sheltered bays, similar to other mounds of this region.
[17] In that case the sinkage would have amounted to about 6 inches, the alluvial increase to about 9 to 10 inches in a century.
[18] The rapidity of the sinkage of alluvial coasts varies greatly owing to local conditions. For the Atlantic Coast the rate of sinkage is 2 feet per century (cf. Abbott, l. c., p. 449). Applying this same rate to the eastern coast of the Bay, we would arrive at the absurd result that the shellmound of Emeryville had begun to form in 1750, while that date was presumably the end of its occupied state.
The Internal Structure of the Mound.
The principal constituents of the mound are the shells. These have nearly all crumbled into small fragments and are slightly mixed with soil, which when damp gives the entire mass the appearance of pure soil. When this is flooded with water the washing away of the sand produces no noticeable change in its volume. This mass has mingled with it bits of charcoal, bones of animals, ashes or cinders, and stones averaging about the size of one’s fist and blackened by fire.[[19]] Marks of stratification may be traced through almost the entire mound. [Plate 5], representing a photographic view of the excavation, shows the stratification planes in the walls quite distinctly. The strata consist of compact masses of more or less fragmentary shells, or of beds of ashes or cinders. In many cases the latter seem to extend through the entire mound. They are sometimes not thicker than a sheet of heavy paper, but show the general direction of the bedding planes, and form a clear contrast with the homogeneous, dark mass of broken shells.[[20]] These planes become somewhat less distinct in the deeper strata.[[21]] As in other shellmounds,[[22]] there were observed certain rounded masses of shells intersecting the lines of stratification. These are caused by holes, made by moles or other burrowing animals, being afterward refilled with shells.[[23]]