If you go back, check over the stones for cupmarks and stuff - there might be something, but the stones will be horribly marked from being tractored about.

If the farm workers are communicative maybe you could ask them whether there's been a crop circle nearby within the last five years or so. It'd be a way of getting aerial imagery.

The dry summer was 1976 - I remember it because of a daughter - and the pond, I assume, would have been a dewpond ? It sounds like your watertable is falling. Just postholes in a pattern is strong enough to give the county archaeologist a call.