How to get rid of many nnml mail directories en-masse

[Harry Putnam]

Over the years I've accumulated many many nnml directories that are no
longer in use.

I know how to delete individual mail directories thru gnus
interface... but how can I delete dozens of them.

Something like just delete them and then excise them from .newsrc.eld?

Is there some way better than that... but doesn't take all day to do?

find ~/Mail/ -type d |wc -l shows 579 directories.  I'm guessing
something like 250-290 are actually in use or at least holding
something I want to keep.  So, I'm talking about something in the
neighborhood of 300 mail directories that need to bite the dust.