I posted this on the other post you commented in but here it is again. That picture looks like they just adopted the holes those holes were probably formed from a river long ago or something of that nature. you didn't take a picture of the bees so i can't tell you if they were mason bees or leaf cutter bees or if they were a more aggressive variety.
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Here is a link that might be useful: TheItalian Garden Mason beekeeping

The holes seem to me to be too evenly spaced and too uniform in size to have resulted from a geological process. They are the kind of feature you would expect to have been created by agents influenced by their neighbors. And there's another similar outcrop nearby with no holes whatsoever. I should have examined the holes more carefully back in February when all was quiet! These bees were flying around furiously and noisily. I didn't want to get close.

The holes seem to me to be too evenly spaced and too uniform in size to have resulted from a geological process. They are the kind of feature you would expect to have been created by agents influenced by their neighbors. And there's another similar outcrop nearby with no holes whatsoever. I should have examined the holes more carefully back in February when all was quiet! These bees were flying around furiously and noisily. I didn't want to get close.

If it's a soft rock - mudstone or other basically packed dirt kind of rock, they can chew into it.