ST,
The plan makes much more sense now.
However, IMO the time to discover your "surprises" is prior to the construction phase. I would highly recommend getting 3-4 samples from your pond site before building. Also, if you have that in hand, your builder will be much more able to give you a bid without a huge range of contingency costs.
One option is to get some drill core samples of your subsoils. Frequently, your county NRCS office can arrange to have a little drilling rig come out and get those samples. (I don't know if you are in the city limits and if that changes the county rules?) Regardless, I would call your NRCS office and see what help they can provide for building a pond.
Even if they don't do that service in your area, it is possible to get it done by a private contractor. However, they will be difficult to find. If you search "soil sampling" you will get a million hits for the service of testing topsoils - which is not what you want.
Another option would be to pay a little "hourly rate" to the contractor you like most to dig a few slot trenches. A large mini-excavator, like a John Deere 60, could get down just over 12 feet. Or use the contractor's smallest "real" excavator to get a bit deeper. He could dig 3-4 sample holes in two hours on your property and get through the sandstone layer to determine how thick it is AND what lies beneath it. You would also have to pay his transportation fee (which goes up rapidly on the larger equipment).
It would be priceless though, to know if the sandstone layer is 20' thick. Or if under that sandstone layer you hit a solid ledge of limestone. Hopefully you will find that you have 2' of sandstone and then go back into good clay-bearing material.
You said the "sandstone junk" had the consistency of crushed concrete. Do you know what "caliche" is? Obviously, I don't have eyeballs on your site, but that might be caliche. If so, it should also function as a "drain", the same as a porous sandstone.
Another thing to consider is your spoils (the excavated material). You can do some landscaping berms (that do not have to seal water) on your property that you core with the unwanted spoils and then put a cover of 6-12" of good topsoil back over. If not, then your pond bid will also have to include hauling and dumping some of your spoils, which will significantly add to your costs.
If your junk rock is caliche, you might be able to use that as rip rap around part of the pond. It is calcium carbonate cemented soil, so it would provide the same water chemistry benefit as limestone rip rap. Do you like the "look" of what you were digging? Crushing it up a little under bulldozer treads will not be as uniform as the material commonly sold as rip rap, but the small fish might like the mixed sizes even better.
Finally, have you talked to any pond contractors that install pond liners? For a 1/8th acre pond, a liner would probably cost about the same as the equipment hours to make your clay seals around the pond. The liner would give you MUCH greater confidence against leaks! (And it might actually be cheaper.)
With a liner, your junk rock layer might even be beneficial. You could slope your edges to a shelf in the rock layer, and then go down at a steep angle to your total pond depth. Your pond would be shaped like a margarita glass. The contractor would then put some soil material over the shallow part of the liner (to grow some plants) but the deepest part of the pond could be bare liner.
Just throwing out more ideas, but you might want to read up on pond liners and see if that might be a good option in your situation where you have a known drain.
