cb100,
The lack of a sealing layer is perplexing on your pond. (Can you post pictures of the "lay of the pond" that shows the dam and the watershed?)
Your pond must have had something that created a seal if it held water well for most of the first 48 years!
One other option is that the sealing horizon has been disrupted by "bioturbation".
Basically, living organisms could have mixed up your sealing layer with the overlying "muck" layer. This could be burrowing crayfish, it could be muskrats, it could be hundreds of small cottonwood trees, etc.
If you NEVER had a sealing layer, then your underlying rock layer must be impermeable and your dam must be "pinned" to the rock horizon with a clay core of some type?
