Is that a limestone or sandstone? It's usually wind and rain that causes erosion into small holes, and small creatures can move in and live there. The webbing in your photo means that a spider has taken up residence.

Oh thank you Landgirl, I thought it might be something horrible.....spiders I like. I've no idea what the wall is built from just that it's old !

Mazzie thank you for the giggle - trying to picture what you imagined might be in there....looks like holes left from a dissolved backbone doesn't it?