The stem wall being constructed here is for an area that I'm calling the "solar gain" room. Facing south, there will be large windows on three sides and a stone floor to absorb solar radiation during the winter months. Once the building is complete a french door separating the solar gain room and the studio, could be openend to allow heat to enter the main studio space. At night, the french doors can be closed and the solar gain room will act as an additional insulating buffer from winter nights that temporarily get down into the 20's and low 30's.
The reason why the stem wall for this room was not completed earlier was to facilitate construction access. This room is a bulge on the south side of the building. If this area was constructed earlier, access to the south & north side of the studio for hauling rock and other material would have been difficult. Trees, cactus, yucca and the grade of the land make access to the south side very tight. Now that the stem walls for the entire studio are essentially complete, construction of the solar gain room has begun.
Technical notes: The stem wall, like the rest of the studio rests on an 18-inch deep rubble trench footing (filed with 1"-3" dia river rock). However, the wall for this area will be only 16"-thick, compared to the rest of the studio where the walls are 2-feet thick.