cb100,
I was typing a longer response, and missed your post that your pond goes down 10-12 feet during the summer. That is WAY MORE than the expected evaporation loss for your area of California.
You are definitely correct that your pond is not completely sealed!
If the material above your rock turns out to be good, packable clay, then you need to spread your material, moisten it to the correct water content, and then pack it in lifts. (There are lots of threads in the forum for the proper procedure, written by some pond-building experts.)
If it passes the hand-packing tests, but you are going to spend a lot of time/money for the renovation, then the next step would probably be a professional soil analysis. You may have a county farm service agency, or similar resource to perform that test for a nominal fee since you are a landowner in the county.
