Photographed while walking past the historic Forest Hill Cemetery* in Madison, Wisconsin. I had to look up some background on the custom.
One of the most common Jewish cemetery customs is to leave a small stone at the gravesite of a loved one after saying Kaddish or visiting...More information at the Jewish Cemetery Association website.
The origin of this custom began long ago, when... the body would be placed in the ground, covered with dirt and then large stones would be placed atop the gravesite, preventing wild animals from destroying the remains.
Over time, individuals would go back to the gravesite and continue to place stones, ensuring the security of the site and as a way to build up the “memory” of the loved one...
Another explanation of this custom is derived from the phrase often inscribed on a headstone that reads: t’hey nishmato tsurura b’tsor hachayim (may the soul be bound up in the bonds of eternal life). Interestingly, the word tsurura (bound) is related to the word tsur, a pebble kept by shepherds in their slings to keep track of the number of sheep in the herd.
Related: This morning while running errands I was listening to podcast #180 of No Such Thing As A Fish. They mentioned a headstone inscribed "You will always be remembered, never forgotten." It had been left behind at a Dublin airport...
* see also Confederate graveyard - in Wisconsin!