The area around Boston (UK) is filled with wetland/marshes that are called fens. when the settlers from that area (the winthrops, say) came to US, they used the same word for the marshy areas around Boston (US).. a large part of the Fens were filled in (as they leveled some of the hills of Boston, in a 1800's plan to improve the city) and the area became a park with a small stream running through it.. 'Fenway Park' which is where they later build a baseball stadium.. of the same name.

(but that info is a YART)- it came up in one of the baseball (ick!) threads last summer or the summer before.

Fen is a norse word, brought to england with the norsemen.