The cavity floor has a couple of rocks. The flat one is too close to the boulder roof to be practical as a work surface.
The granite in this area is foliated, and breaks into neat slabs. A side view of the boulder shows two such slabs, which have slipped off. There are no tool marks visible along the edges of these slabs. Presumably, the cavity was formed by chipping away the lower layer of rock.
I couldn't find any grinding slicks at the other propped boulders in the corner. Instead, I found evidence of quartz quarrying near the hilltop site with the large pedestal boulder.
Note the propped boulder in the left foreground. It split vertically, producing the flat slab my knapsack is resting on. About 140 feet from this site is this boulder.
A careful look inside reveals this large chunk of quartz, left from a vein.
In front of this little quartz mine.
Maybe the table site was the men's stone work area and meeting place. Even though the granite rocks seem permanent, they are slowly splitting, breaking, and shifting. These sites are only shadows of their original structure, and are vanishing.