You can rent a small trackhoe and dig down to 10 to 15' to search for clays. I'm no expert but I have done this. It's not hard to do. Suggest you buy Mike Otto's book on building ponds. you can find it here on PB. I would also go deeper than 8' if you can because of the droughts we all go through from time to time. We have been going through one for the past couple years here just west of Shreveport. An 8 foot pond might be a 5 foot pond in those cases. And Welcome to the PB Forum. An expert should along shortly.
Do not judge me by the politicians in my City, State or Federal Government.
Thank The Good Lord the government in Washington DC gets little done.
Outlawing guns will make a lot of us down here in the South
Outlaws and proud of it