Pounding vs. Grinding. Different ancient activities left distinctive evidence behind. The bedrock mortar surface at left shows hundreds of tiny fracture lines within the individual crystals that compose Enchanted Rock granite (quartz, feldspar, biotite and muscovite among them). These fractures were created by countless pestle impacts to the mortar surface as the mortar was being used to pulverize foods into flour or meal. (The scale units are centimeters.)
In contrast, the bedrock metate surface at right (shown a slightly higher level of magnification) shows a much different wear pattern. Here, a single pink feldspar crystal has been sliced and polished by grinding. Pitting around the edges of the crystal suggests that some smaller crystals have been removed by the grinding process, but the impact fracturing so typical of mortar surfaces is completely absent. These contrasting wear patterns allow clear distinction between grinding surfaces (metates) and pounding surfaces (mortars), even when they overlap on a single bedrock bench. Photos by Dan Potter.