First the rocks; it looks like you've got a metasedimentary conglomerate(a sedimentary rock that has been partially metamorphosed) and chert(kind of like flint)
For the clay;
First perform a sediment test with a mason jar and water, measuring the height of each layer. Fill the quart or larger jar with soil and water, then add 1 tsp salt attach the lid and shake vigorously. Allow to stand for 2 days in order for the clay to separate from the silt. Measure and this will give you rough percentages.
Second is the clay test;
Make a 1/2" roll of soil, like a sausage, it should not be too wet and sticky. Roll it out in your hand to 1/8"-1/4" and measure how long the thread will build before it breaks.
10-12" high clay
2-4" with difficulty is low clay
Now, since it looks like you've got pretty good building soil, mix in the straw etc. and try different hydration rates on a ball of mix, dropping them from shoulder height onto the ground. The splats should mostly hold together, but break into a few heavy pieces. One piece-too wet, lots of pieces-too dry. Now test the mix on a sample wall. Test every batch of mix this same way and you're ready to go.
Kim Bowen wrote:I have seen a lot of cob in pictures. It always looks brown or reddish. I have not seen any cob with black clay. It might turn out to be pretty ugly. But the option to put a lime plaster on it seems likely. In the jar test it was really evident how dark the clay and sand both are. In my mind I am wondering what geographical event would have caused this varied soil types.
I don't know where you live, so it's difficult to say. The photos of rocks indicate that you have a fair bit of volcanic activity and that the clay is probably from hydrothermal/chemical alteration of, here I am guessing, granite(indicated by the mica in the clay).
Let me k ow your geologic area and I might have more guesses for you.