I was out for a walk at lunch and passed by the Holocaust Memorial. I learned way back when about the stones you see here, and I love the idea and sentiment behind it:
According to a Jewish friend of mine, it is Jewish custom to leave a stone so that future visitors will know others had been there. I also read that its origins are in the fact that ancient tombs consisted of large stones piled upon each other. The practice of putting the stones on someone's grave indicate that one is helping build a monument to the deceased.