Steve,
I have done the soils investigations on hundreds of sites over the years and have four ponds built in valleys on my farm. Two of which were built by others and leak. Soils maps can be very deceptive since they only show surficial soils, typically 18" deep. Valleys are dangerous because often there are alluvial deposits inclding sand and gravels in valleys.
You or are the right track with your "quick and dirty" permability test. Dig a couple of test pits in front of and behind where the dam will go. Look for sands and gravels. An experienced contractor can guide you in the right direction.
