When you visit a cemetery, you may see small rocks or pebbles set on top of monuments. This stems from a Jewish tradition. The predominant interpretation of the practice of leaving small stones on grave markers is that it originated in a time before modern burial and tombstones. The body would be prepared and wrapped, and then buried, but the arid heat of the Middle East, it was difficult to bury bodies deeply. A cairn of stones would be built over the burial to prevent animals from disturbing the grave. Leaving rocks at each visit would not only serve as a signifier that someone had visited, but would literally build and maintain the memorial. Even though the funerary practices have changed, the ritual of leaving a stone has survived and in some cases grown. I know a number of cemetery visitors who have left stones on graves who are not Jewish or even familiar with very much of Jewish religion and culture.
I found both of these heavily covered markers in Lake View last fall.