what type of boundary marks the border between New york and connecticut

Looks like an artificial (man-made) one to me, not a river or any natural landform:

http://www.google.com/search?q=new+york+connecticut+border+map&es_sm=119&source=lnms&tbm=isch&sa=X&ei=qmchVKrxIc2ZyATE-IHwCw&ved=0CAoQ_AUoAw&biw=1440&bih=707#facrc=_&imgdii=_&imgrc=0CK3cQzQpqLMIM%253A%3BcWA6fgUraCgnHM%3Bhttp%253A%252F%252F3e3utw244xu62y7d8u2ogrlf146e.wpengine.netdna-cdn.com%252Fwp-content%252Fuploads%252F2013%252F09%252FCT-county.jpg%3Bhttp%253A%252F%252Fwww.mapofus.org%252Fconnecticut%252F%3B1600%3B1118