Effects of Erosion

I have been wondering recently about why the facades of certain headstones crumble away to nothing. I thought I would look at the effects in more detail. Here is the first stone. While the weeping willow and tomb at the top of the headstone are fine, the top third of the inscribed lettering has fallen away. No trace of it exists so it must have been disposed of. I hope gravestone recorders collected the information before it was lost. The lower part is close to falling away.