Well, I spent the better part of a day searching for answers about this soil, and here is a rough summary of what I found out so far:
Regarding the picture I posted of the other nearby ponds, I was able to talk with the owners of the largest pond. They had it dug ca. 27 years ago and it is over 20 feet deep! The topography was a gully (like our site), with the same Weikert shale and silt. When I inquired about the embankment dam and needed clay, the owner mentioned that the contractor found clay below the topsoil and above the Weikert shale. In fact they had enough to do the entire dam. There are no feeder streams, just watershed. He did mention that it does seem to leak some, but once it goes down to a certain level, it stays put.
With this news in hand, I went home and grabbed a shovel to see what lay beneath. I dug right about where I estimate the core of the dam will be. The topsoil is some typical silt, about 4-6" thick. Right beneath it (at least where I was digging) was a strip of very nice clay that ran about 4" thick. Here is what it looks like when I rolled it out. As you can see, there is quite a bit of plasticity--which is a good thing:
Directly beneath this strip of clay is the Weikert shale I had heard so much about. The stuff really is not as bad as I had expected, though left uncompacted it surely would drain. But notice what it does when it is semi-moist (but not too much):
It held a ball form pretty good, though certainly not as good as the clay. Moreover, it has to have some water to compact like this, but not too much, otherwise the fine materials disperse and you are left with a handful of aggregate.
Encouraged by this, I dug a hole about 2.5-3 feet deep. At that point water was seeping in through the sides at certain points (the soil is still wet down there from rains the previous week). So I filled the hole to the top and went back to mowing:
Here are some shots to give you an idea of what I have in mind in terms of the layout. This first one is toward the rear of the proposed site, with a line drawn across to give an idea of where the core/embankment would be. Down the middle is a depression where the seasonal run off flows (it is a small creek right now, but it dries up):
The last pic shows the gully from above. It really does not do justice to how deep down this gully is.
If this first pond works out, on the opposite side of the driveway is the feeder spring (which flows through the culvert). This would be a beautiful spot for another small pond (it has alot of nice shade trees. Good place for the kids to catch frogs):
Just to the left of the spring is an old abandoned hand-dug well (which is currently full of water), which I will either excavate with the second pond, or just fill in:
I did speak with two more pond owners up the road--one creek-fed, the other run-off (the ponds that is). They pretty much inherited the ponds, but gave some good drought and wet-season observations that will prove helpful. My Tim Matson pond book should be in the mail any day now. I read it in the past (library copy), but decided to get one for myself. He has alot of inspirational ideas.