Habitual thinking has perhaps contributed to the general idea that a lithic assemblage of core tools characterized by percussion flaking has come to represent both antiquity and a hunting- or skin-dressing economy. This may be true in instances where the physical and the biological evidence are sufficient to support such a claim. The general antiquity of the Tank Site can be established on the grounds of physical evidence but the great quantity of core tools invites speculation to account for their occurrence in a nonhunting culture. Some 4,994 core tools were collected from the Tank Site of an estimated presence of 50,000. This figure is exceptionally high, compared to other southern California sites of assumed similar age. Greatest in amounts were scraper planes and core hammerstones. Curiously enough, none of the scraper planes, regardless of finish, shows any degree of wear of battering on the presumed working edge or polishing on the under surface as would be expected to occur with use. This is true of similar artifacts from the San Dieguito, La Jolla, and Lake Mohave cultures. Possibly as a fleshing tool no such wear would result, but evidence of skin dressing is lacking with so little mammal bone occuring in the site. A wood-working or plant-fiber economy likewise would hardly necessitate any great quantity or variety of tools. The large pulping plane used to remove mescal fiber from a leaf was rare even among the historic Diegueno. To account for the large number of "core tools" in the Tank Site, especially scraper planes, choppers, and core hammerstones, it is suggested these tools are actually crude implements used in the manufacture and maintainence of more refined implements directly associated with the mass production of the food economy, that is, the milling stones known as the mano and metate. The function of these primitive mills is to grind, necessitating a rough contact between the surface of the mano and the metate. When the surfaces are worn smooth, caking of meal (acorn) occurs on the polished faces, resulting in the reduced efficiency of the mill. Primitive peoples who have used the mano and metate solve this problem in one of two ways. In the Southwest and in Mexico volcanic scoriaceous or vessicular basalt is selected for either or both the hand stone and the grinding slab. As wear progresses new holes are constantly opened up on the surface of both the grinding tools and hence the mill constantly maintains full grinding efficiency and is self-sharpening. If granites, sandstones, or schist are used, as is often the case, then on frequent occasions the grinding surfaces have to be artificially pitted. To do this a core is made containing angular points and edges and sharp blows are struck on the faces of the mano and metate, removing small pitts (fig. 5, _b_, _f_; Treganza and Malamud, 1950, pl. 22, _b_, _c_, _g_). This latter technique of pecking or crumbling is the