Early millers struggled to obtain suitable millstones because grinding required exceptionally hard rock that often was not locally available, so many stones were hauled long distances by ox-driven wagon from specialized quarries where rock with the needed qualities was found. Shaping millstones from raw rock demanded tremendous labor, and stones with flaws were usually discarded during the process. The earliest American grain mills used millstones imported from Europe, including French buhrs made of pieces of stone from famous centuries-old quarries near Paris, France. Some millstones were made from hard sandstone, but many were made from conglomerate, a pebble-filled rock also called hailgrit or pebble grit, which, though less effective than some imported stones for producing high-quality wheat flour, was well suited to grinding animal feed and corn meal, an important pioneer food.