Or, the polarity changed in situ over thousands of years to reflect the polarity of the earth
I agree with your assessment. ie, Induced magnetism.
Bingo. Especially in the 20th century, many motors and electric and electronic equipment as been around them. There is high probability that the magnetic pattern is of recent adjustment.
“Or, the polarity changed in situ over thousands of years to reflect the polarity of the earth”
Polarity in rocks is based on the alignment of molecules before the rocks solidified. It is possible that a very powerful magnet could realign the ions, but I doubt the field from the magnetic poles would be powerful enough to do that.
If that did happen, though, then why wouldn’t ALL the blocks be realigned? Why would they find a couple of random blocks that were aligned differently?
It doesn't work that way. When in the liquid state, the magnetic field is imprinted, after solidification, it doesn't change.