Vanishing graves!

This plot with plenty empty space has graves from the 1940's '50's, '60's
and a few from the early 1970's. I am a regular visitor here yet I am
  amazed by the way graves just vanish without me evening noticing!
 India, like countries in Europe,
where land is in short supply, no more land is being provided for burials.
The picture has been taken diagonally
so the plot to the left half of the pic, beyond the trees, is the plot
with a few graves from the 1960's, '70's and some from the 1980's.

 I don't remember these black stone graves so I cannot
say whether they are being redone. This plot was filled up
starting from the 1990's replacing those from the 1920's and
there weren't any empty spaces till a few months ago!