The site is centered on a draw that runs through the center of our property. It only has about 5 ft of elevation difference over the area that I'm planning to build the tank, so I'll have to do a fair amount of excavation to get a 10-12 ft depth. I believe the soil (mostly black dirt) has an adequate amount of clay (this particular area stays very wet for weeks after a rain), but I am a little concerned about finding rock underneath. This draw eventually empties into a creek on our property, and that area full of shale. There is another tank on an adjacent property on the same draw (above mine) and it seems to hold water very well. Wish you were closer...