Rock plant

I am not sure what this self-set plant is but am guessing it will be some form of saxifrage or sedum.  What I find remarkable is that it appears to be growing straight out of the limestone.  I don't know how it survives but its roots must have found some microscopic fissures somehow.

Edit:  I now think it may be Chiastophyllum oppositifolium aka 'Lamb's Tail'