I went through a cemetery today, and there were rocks on some of the Jewish tombstones. Why were they there?

I think it's mainly traditional currently and I cannot remember the reasons behind it. I just had to post cause your post brought back memories I hadn't thought of in years! I remember doing this as a child (4-5yo) when visiting my grandmother's grave!

http://judaism.about.com/library/3_askrabbi_o/bl_simmons_stonesgraves.htm

We are taught that it is an act of ultimate kindness and respect to bury someone and place a marker at the site. After a person is buried, of course, we can no longer participate in burying them. However, even if a tombstone has been erected, we can participate in the mitzvah of making a marker at a grave, by adding to the stone. Therefore, customarily, we place stones on top of a gravestone whenever we visit to indicate our participation in the mitzvah of erecting a tombstone, even if only in a more symbolic way.

Thanks Barb.

DH tells me that they leave stones on Thoreau's grave, too.

What a neat custom! Glad you posted this.