Note
I'm looking at the various reference materials I had available to me yesterday along with the comfort of home, no mosquitos, and lots of internet. I was indeed off the map to the west, near Rocky Hill Rd. I was near a model aircraft airfield, with nicely mown grass like a putting green. I was unable to google it yesterday from the field. "Oxford Ma model airplane club" (what I googled) did not find the Rocky Hill Flying Circus (which it actually is). The quarry nearby has changed the land forms somewhat since the topo maps were made. The Midstate Trail hiking guide had several problems that did not help me get unlost. The map in the guide obscured the word "French" along the French River. I was trying to direct someone at home to consult that page of the guide and see where I was relative to known parts. The Midstate Trail has also been rerouted in places, and it's unclear why the trail has been rerouted, which blazes in the terrain are new or old, and which variation the Midstate Trail guide corresponds to. The breathtaking cascades that I found were not called out in the Midstate Trail guide, either.
[Maybe] it's time for me to carry pump+spares on my mountain bike.