Indian Grinding Rock State Historic Park
A single limestone slab holds the evidence of countless meals prepared by Miwok hands over many centuries.
Each of the 1,185 mortar holes represents hours of grinding acorns with stone pestles, turning them into edible flour.
Here’s the story of this ancient food processing site that became a California treasure.
The Miwok People
The Northern Sierra Miwok have lived in this area for many centuries before Europeans arrived.
They built their homes along rivers and streams throughout the area, and their villages spread from the Consumnes River down to the Calaveras River.
Acorns in the Miwok Diet
The Indian Grinding Rock area has open meadows with three types of oak trees that dropped plenty of acorns each year.
They could be stored for long periods, making them a reliable food source during winter and times when other foods were hard to find.
The Formation of Mortar Holes
Generations of Miwok women ground acorns and seeds using stone pestles against the limestone, slowly wearing cup-shaped holes in the rock. All of those 1, 1850 depressions represent thousands of years of manual effort turning acorns into food for their families and helping the tribe survive.
The Petroglyphs Among the Mortars
This grinding rock also has 363 petroglyphs carved among the mortar holes. These carvings show circles, wheels with spokes, animal tracks, human footprints, and wavy lines.
It’s the only known place in California where people decorated their grinding rocks with carvings.
Some of these carvings are 2,000 to 3,000 years old, and many have faded over time and are hard to see clearly today.
How Acorns Were Harvested
The Miwok gathered acorns in September during “Big Time.” This yearly event brought Miwok groups from the Bay Area and Sacramento Valley to the grinding rock.
People knocked acorns from tree branches using long poles or sometimes climbed the trees. After collecting the nuts, they dried them in the sun or near fires to keep them from spoiling.
Removing the Bitter Tannins
Raw acorns contain tannins that make them too bitter to eat, so the Miwok had to remove them with water.
After grinding acorns into meal, women poured hot and cold water through it to wash out the bitter taste. They made special basins in sandy areas for this washing process.
They usually needed about ten rinses with water. Women tested the meal by tasting it and checking its color, properly washed meal turned from brown to white.
Cooking Acorn Meal
After washing out the bitterness, the women mixed the acorn meal with water in tightly woven baskets.
They heated rocks in a fire until very hot, then used wooden tongs to put the hot rocks into the acorn mixture. Moving the rocks with wooden paddles kept the mixture from burning.
This method of cooking with hot rocks turned acorn meal into mush, soup, or bread.
A Community Kitchen
Many women worked simultaneously at the grinding rock, essentially turning it in a huge communal kitchen.
And like any kitchen, it became the social heart of the settlement.
As the women, they talked about marriages, births, deaths, and community events while grinding acorns. Children played nearby as their mothers, aunts, and grandmothers worked.
It’s also where the grinding technique got passed down to the younger members of the tribe.
The Smithsonian Replica
The real limestone rock at the Indian Grinding Rock State Park park is very fragile. Weather and erosion are slowly wearing away both the holes and carvings.
Because of this, visitors must stay off the grinding rock and look at it from the walkways and viewing platforms instead.
You can also check out an exact copy of the Chaw’se grinding rock at the Smithsonian Institution in Washington D.C.
Visiting Indian Grinding Rock State Historic Park
The park is located at 14881 Pine Grove-Volcano Road, Pine Grove, CA 95665. It’s open daily from sunrise to sunset, with an $8 vehicle entry fee. The Chaw’se Museum operates from 10 a.m. to 4 p.m.
You’ll find the grinding rock, reconstructed Miwok village, and museum as main attractions. Don’t miss the 60-foot ceremonial roundhouse or the two hiking trails.
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