Querns (also known as mortars, milling slabs, motan, millstones, and metates among other things) is a term archaeologists use, and some normal people too, for the bottom half of the pair of stone objects used to grind things. The shape and size of the quern varies greatly over time used and eventual function. Some are flat deep bowls or "flower pot" mortars, designed to hold the flour in a puddle at the bottom: others are flat slabs, meant to spread out the flour as it is ground.
Querns are quite often very heavy indeed, and the earliest form identified is simply an enormous slab of stone that has a ground concave depression in the upper side. Some are elaborately carved objects with rims for the grinding surface, feet to bring the object off the floor, and sometimes have a built-in tilt, such that the flour rolls downhill as it is ground small enough.
Some mortars are small enough to be held in one hand; others are too large to be moved from site to site and are left behind when mobile hunter-gatherers shift camp. (see Buonosera 2013).
Sources
Buonasera TY. 2013. More than acorns and small seeds: A diachronic analysis of mortuary associated ground stone from the south San Francisco Bay area. Journal of Anthropological Archaeology 32(2):190-211.
Also see the bibliography.
