I think I might be getting really, really picky; but, I'm trying to be consistent.
There is a geologic event called the Taconic Orogeny. This is when the Appalachian Mountains were formed. (They are called the Taconics in Connecticut.) I have just encountered a handful of people calling it the Taconian Orogeny. After doing a Google search, I have a theory that it might be an adjectival form used by people who don't speak American English...or just grammatical slovenliness.
I have no clue what the adjective formation rule would be here. I would like some kind of rule so I can impose grammatical order on some Geotechnical Engineering Reports.
Can anyone help?
PS: In case this helps, this site
http://www.geography.hunter.cuny.edu/bight/highland.html will explain the Taconic Orogeny. Caveat jargon.