My first contractor did not core deep enough and my pond leaked.My new contractor went further down and hit limestone.The rock had cracks with sand in between and some big crevices. We had to dig this layer out to the hard solid rock below. We dug up a layer of sea shells 6" to 8" thick and a layer of porous rough green sand between the limestone and solid blue rock. Here are a few pics but they don't do it justice.
