What is below the 60 inch mark? To properly seal the bottom of a pond, ideally you should have a 2' thick layer of clay, compacting it with a sheepsfoot roller in 6" lifts. That blue clay is good stuff.
Honestly I have no idea, it could be bedrock or gold for all I know. Hopefully I won't have to excavate too much (The land is actually a sloping valley between 2 mountains)
What I do know is that on this same land about 30 years ago my dad tried to build a small spring-fed excavation pond for his horses and cattle. The guy brought a small dozer in and went down a few feet. He freaked out when he hit the blue clay layer, and refused to go any further saying he would get hopelessly stuck.
Anyways the old pond ended up being about 1/8 an acre and even though it was only halfway built it managed to hold water until silt filled it up and the old the spring dried up.
