I wonder how much the soil can change within the proposed span of your pond. It might not be a bad idea to dig multiple test pits around the proposed pond site. If you hit an excessively rocky area, it might be best to avoid that area.

I really know nothing about building ponds, but if you have concerns about holding water, I would think that making a clay liner would be the most sure thing.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."