OT - Jewish Burial Practices / Traditions

[Littlearmies]

Hi guys,
I just came back from 10 days holiday on the Somme. Obviously we visited a few cemetaries and I noticed that the gravestones for Jewish burials (the graves are all side by side with no distinction between rank, nationality or religion) had a few pieces of dirt or rock left on each one.

I'm sure I could google this info but figured there would be someone who'd know here.

Regards
Malcolm