I have the answer...During the last ice age..the ground was buried by a glacier several hudred feet thick.. The locals slid the blocks out on the ice..little effort involved..then waited hundreds of years while the ice melted, so they could easily position the blocks atop each other on the ground, knowing that thousand of years in the future people would go bananas trying to figure it out..
"dude, hold muh ale..."?
Not necessarily a joke. There was a civilization prior to ours. The ice age seperates time. 10K years of entropy leaves no trace. Except for a "?".