Here's the cornerstone of our new extension. I needed to place it so I could have a more tangible datum point from which to take measurements. Mason's line is alright, but it can be hard to get everything square with just a bit of string.
The fact that we couldn't get a normal poured concrete footing (from a normal concrete mixer truck) placed in such a crooked hole meant that I had to send back about half the block I ordered and instead get bagged cement. I also rented a much larger cement mixer.
We'll have to struggle to custom fit the footing to the contours of the ledge, a few feet at a time, pinning with rebar as we go, making eight inch steps to take in each rise of block. The big cement mixer will make lighter work of making that much cement by hand.
Luckily the ledge is nearly flat and solid for about one third of the length of the foundation wall, and has a well-defined natural rock trench for about another one third, so the amount of concrete needed is much less than for the planned footing, which was going to need three yards. I hope to get by with less than two now.