GUYS! I am pretty sure those little Stonehenge references are are just humor. The author is not REALLY suggesting that future civilization would be puzzled by Manhattan's grid. The comparisons with Stonehenge are just an amusing way of pointing out this phenomena that many people are probably not aware of.
Personally I think it is pretty cool that the perfect conditions for observing this are only available in a few cities - despite the fact there must be a lot of cities around the world with skyscrapers and grid like street plans.