Wiktionary
post mold
n. (context archaeology English) Evidence of a post (presumably of wood) hole location, discerned during archeological excavation from differences (color, texture, constituents, etc.) of soil in the cavity compared with surrounding matrix soil. The cavity, serving as a mold for the soil and debris (the fill soil) from ambient natural erosion and weathering, became filled over time, indicating the approximate shape of the erstwhile post.