Originally posted by Zep Tepi:

People have been known to find the site before ever finding a stone leading to it.

I have done that several times. I have been very luck lately, randomly coming across a stonehenge when I wasn't looking for one, and having it contain the marker. I believe the smaller (3 stones in an arch) formations can have them as well.

I suggest just sailing around plains and looking for stone formations and/or the alter itself. Hop out and check all the stone formations you come across, just watch out for berzerkers!