Oh, I almost forgot...
If you want to do a quick & dirty (literally) evaluation for yourself, shovel down a few feet and grab the soil. Squeeze. If it is good clay and has any moisture, it will retain its shape after your stop squeezing. It will also feel somewhat tacky. (Don't let your wife see you doing this, especially if she has a camera handy.)
If it quickly falls apart, even with some moisture, it won't hold water for a pond.
Technically it's called the Permeability Index, the degree to which the soil allows water through. The higher the PI, the better it holds water. The government recommends a minimum PI of 18 to build a dam, but good pond builders usually want something over 25 as a safety margin. Mike Smelley, the dirt guy who actually built my dam, said the gray white clay on my property probably had a PI near 60, about as good as it gets.
