tezoquitl.

“...teçoquitl, heavy clay (Sahagún, 1963, p. 252).... ” (p. 54)
Barbara J. Williams, "Pictorial Representation of Soils in the Valley of Mexico: Evidence from the Codex Vergara," Geoscience and Man 21 (1980), 51–62.

“Ethnographic data suggest that heavy clay is the appropriate translation of the stone-spine glyph. Nahuatl-speaking campesinos in the area refer to the clay soils as tezoquitl. Clays and clay loams dominate the valley floor around Tepetlaoztoc today and probably did in the sixteenth century.”
Barbara J. Williams, "Pictorial Representation of Soils in the Valley of Mexico: Evidence from the Codex Vergara," Geoscience and Man 21 (1980), 51–62.

In the glyphics, “the angle at which the spine pierces the stone does not alter the meaning of clay (teçoquitl), nor does the orientation of the stone.” (p. 58)
Barbara J. Williams, "Pictorial Representation of Soils in the Valley of Mexico: Evidence from the Codex Vergara," Geoscience and Man 21 (1980), 51–62.