Some one else just posted essentially the same question:
https://www.helpfulgardener.com/forum/posting.php?mode=editpost&p=144215
Here was my answer:
If you type root over rock into the Search the Forum feature, you will find tons of listings.
This one has a link to an external article about how to do it:
https://www.helpfulgardener.com/forum/viewtopic.php?p=101104&highlight=root+rock#101104
here's one with a bit of step by step photos:
https://www.helpfulgardener.com/forum/viewtopic.php?p=136778&highlight=root+rock#136778
here's some more step by step
https://www.helpfulgardener.com/forum/viewtopic.php?t=20201&postdays=0&postorder=asc&highlight=root+rock&start=0
