Originally Posted by Jordan G

Yeah - what area is that from?....is this typical throughout? Do you have another pic of any good, intact stone?

It really looks like surface delamination/spalling from a "sandy" type stone, unless I'm just seeing a patched area. The surface of the weak, permeable stone could very well be wicking moisture in and the freeze/thaw action is aggravating it. Your mortal joints actually don't look too bad. My gut reaction would be to patch the voids and seal with a sika product or boiled linseed oil?