I took this picture of a very large moss covered stone at Henninger Flats.  It has a hole in it as if it was used by natives for grinding.

It measures about two feet square and is about a foot and a half high.  During the rainy season it has this very nice look.  In the summer it is just a stone with a hole.

To find the stone hike up to Henninger Flats from Eaton Canyon and look for the water point behind the first building on the left.  You can get more information at the MountWilsonTollRoad Informer web site