Having dug out more than half of the length of the bed, we have exposed the foundation of the house. While it may be sitting on rock further under the house, around the edges, it seems to be a mix of rock and dirt. As with much of the rest of the house, it seems reasonable to seal this off with a cement wall, to keep trees and other plants, or animals, from getting under the foundation.
Using an on-line concrete calculator, we figure we will need about 24 bags of 60-pound concrete. Two trips to Home Depot gets us 24 bags for $72.48.
Next we frame up along the foundation.
And after two days of pouring concrete and letting it dry, we can take the forms down.
Now it's back to digging out dirt and rocks.