Listed on the national register of historic places, the Indian Grinding Rock State Historic Park is located near Pine Grove, California & features 1,185 mortar holes made by the Miwok people dating back a few thousand years ago.  The main items grounded were acorns from the native scrub oaks as well as other seeds for cooking.  In addition to the mortar holes are 363 petroglyphs embedded within the marble depicting animal tracks & wheels however most of them have faded over the eons.  A reconstructed Miwok village is part of the park today, featuring teepees & a roundhouse.