A recent CNN article points out that the Georgia Guidestones, a carved granite monument erected in 1980 by a mysterious donor obsessed with the possibility of civilization's destruction, wouldn't be all that useful to humankind's survivors:
The center column has a slot through which the transit of the sun throughout the seasons can be observed, while a hole higher up focuses on Polaris, the north star. Another hole in the capstone focuses a beam of sunlight onto the central pillar at noon. Those features would allow the survivors of Christian's feared apocalypse to reproduce three of the basic tools of civilization: the calendar, clock and compass.
Loris Magnani, an astronomy professor at the University of Georgia, questions how useful the Guidestones would be to survivors of civilization-ending cataclysm. The devices incorporated into the stones are "relatively easy stuff" that most human societies have developed early in their histories, he said.
"Don't get me wrong. As a monument, it's fine. There's nothing wrong with doing that," Magnani told CNN. But he added, "Every decent civilization going back to a couple of millennia before Christ has figured this out. How to make gasoline? Now that would be useful."
Unfortunately, lack of utility hasn't protected the monument from being vandalized by religious/conspiracy nuts. Because it would be so terrible for post-apocalyptic humans to reinvent the clock, and stuff.