The paved part of Snake Hill Rd in Ayer ends just shy of the hill at a Rod and Gun club. If you park there and head west into the woods, there is a lake about 1/3 miles away, surrounded by houses. Just before the lake and houses there is a last bit of bedrock hill and the highpoint of that bedrock is a place where, as you turn and look around you, there are about 8 different horizon directions marked by a small pointed standing stone poking up over the near horizon.
These pointed stones are inconspicuous and the site is not too impressive but I suspect this was a calendar site.
Nearby were other small knolls which could have been auxilliary sites and there were a number of large rock-on-rocks which might also have been playing a role in marking the horizon. So it appears that some rock-on-rocks might be built as simple horizon markers.