Before European settlers arrived in New England, the Nipmuc people populated this area nestled within the Concord and Sudbury River valleys. The large granite boulder (six feet in diameter), was used to grind corn, grains and nuts into meal or flour. The material to be ground was placed on the large concave surface and a stone pestle (usually a hand-sized, smooth stone) was used to pound and grind the material until it was a uniformly-sized meal or flour.
For the most part, each household in a village used their own grindstones and large, shared grindstones (like this one) were not very common. Of the six known grinding stones in Sudbury this is the largest. All the stones in Sudbury have a "seat" — a secondary surface on one side (what this surface was used for is unclear, but perhaps it was in fact a “seat” for someone else to rest against). These "seats" appear to be unique to the stones in Sudbury.
While looking at the satellite map, this hide appears to almost be in someone's backyard. While the visit does, indeed, take place in a residential neighborhood, all activity occurs at a well-known town historical site. However, given it's residential nature, I do not recommend visiting at night.
The cache is a small hide-a-key container hidden about 25 feet away from the stone. it is NOT winter-friendly. Please be respectful of this historical area and replace the container as you found it.
Congratulations to gojeep and kestrel76 for the co-FTF!