I came across this blog:

How does nature overtake a cemetery? Maybe the last living members of the group in charge die, or the families with relatives buried there move away. Perhaps a few people — or even one individual — cared for the site and then one day stopped showing up for who knows what reason, like in the case of Philadelphia’s B’nai Israel/Hebrew Mutual Cemetery. Then there are those cemeteries under the care of an organization whose age simply means its gravestones have weathered centuries of, well, weather.

Read on and look at the photos on the author's website...