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]]
In some shellmounds in other regions strata of earth and sand were found between the shell layers. These give evidence of a temporary evacuation of the shellmound. No evidence of this character was obtained in the study of the Emeryville mound, where the only occurrence of a natural vegetable soil is the surface cover of 1 to 2 inches in thickness, which has formed since the mound was finally abandoned.[[24]] It is possible that slight differences in the state of preservation of the shell deposits which now mark the strata lines may have been caused by differences in the length of time of occupation. Other explanations might, however, be offered.
The lines of stratification mark clearly the gradual development of the strata of the mound from the base until the present truncated cone was formed. It is apparent that two different principles governed the growth of the mound. At certain periods it tended to take on a shallow plateau form. At other times a conical shape developed without the corresponding increase around the base. According to the first principle the mound grew in the form of a plateau to a height of from 9 to 10 feet. Near C in [pl. 4] the edge of the plateau still seems to be traceable, from which point the strata inclined downward. At that period the mound resembled in its proportions the old Indian camping places of the interior valley, some of which are still occupied; or some of the shellmounds along the Bay which have been abandoned at some earlier period. The undulating lines of the strata, such as seen near f and g, suggest irregularities of the old plateau surface, similar to those which may be observed in the surfaces of camp locations of the interior, which have been abandoned for decades. The hollows from 20 to 40 feet in length mark the sites of former sweat-houses or council-halls; these curves, such as that from f to g, may have a similar origin.[[25]]
The manner in which the mound was occupied for habitation varied in the upper strata. With the growth of the mound the diameter of the plateau decreased instead of expanding. From line b upward the strata incline obliquely toward the sides. This change in the manner of forming the mound signifies a change in the character of its occupants. It would be interesting to determine, if possible, the exact line where these two types of growth have met. It might have been about 12 feet above the base, so that the mound grew in the shape of a shallow plateau as far as the middle of stratum V in [pl. 4], and that it changed after this period to its conical form.
[19] The descriptions of nearly all the shellmounds explored in other parts of the world tally exactly with this one; cf. Ranke, l. c., II, p. 532, for the Danish Kjökkenmöddinger; Schumacher, on the general similarity of shellmounds of the Pacific Coast with the mounds in Denmark, Smithson. Rep., 1874, p. 355, etc.
[20] Although no shellmound is free from stratification marks, owing to the gradual growth of the strata, Brinton maintains that this is the case with shellmounds on the Atlantic Coast (Smithson. Rep., 1866, p. 356).
[21] Compare the interesting observations of Wyman (Amer. Naturalist, I, p. 571) concerning shellmounds of New England, that there the shells of the lowest stratum were softer and more crumbled than those of the upper strata.
[22] Cf. Wyman, l. c., p. 365, on a shellmound in the vicinity of Portland, Me.
[23] Similar holes made by moles may be observed occasionally upon old shellmounds along the Bay, which if they had been refilled with shells might also have assumed a rounded form. In such a manner may be explained the finding of a modern steel knife, with the wooden parts still well preserved, in one of the strata of the shellmound of West Berkeley in a place to all appearances undisturbed.
[24] Cf. also Wyman, l. c., p. 571. The absence of true soil from the interior of the mound is proof that at no time was the mound abandoned by its occupants long enough to allow of the formation of such a stratum.
[25] Somewhat smaller but quite similar hollows are still preserved upon the surface of the shellmound of Ellis Landing, and are doubtless sites of houses of that nature.