I hope this list does develop. I love lost/obscure language.

I did a quick search for Geography Dictionary and found this...

http://www.babylon.com/define/48/Geo...ictionary.html

...which had a nice definition of Fen in it. As well it led to http://www.babylon.com/free-dictiona...ngs/44278.html which allows you to browse through the terms.

I've seen a few dictionaries of geography over the years. Perhaps finding an old one via Google Books might be worthwhile.

One of my favorites sites is: http://phrontistery.info/

Perhaps it's worth contacting the site owner to see if he has any related list or suggestions for you. It sounds right up his alley.