The town of Duxbury has a few different conservation areas but most people who hear about the Duxbury Conservation Land (or town forest) are interested in the North Hill Marsh area located off Mayflower Street. Skunkty14 has already provided a Google map to find the parking area but you might also be interested in a trail map too.
The Duxbury town website has maps listed in the conservation area of their website. I would have put a link to the maps but I do not have the required number of posts (yet). To get to the maps, you will need to open the "departments" tab, then the "conservation" tab. At this page there will be a "conservation areas" tab that has the maps.
The North Hill Marsh map is a hikers trail map which will give you a good idea of the area. Unfortunately, it does not show all the trails that are available for bikers. I suggest you park in the lot that would be on your right if you were coming off of Route 3a. If you ride north on the highly used double track past the signboard and the rocks you will find a very nice single track on your left about fifty feet past the rocks. Follow this trail to a small cranberry bog (on your left) and look for the trail that is going up a hill on the right of the bog. This trail will lead you out to some real nice single tracks.
If you crossed Mayflower Street, that portion of the Conservation Lands is called Round Pond. A map of these trails is also posted on the site I mentioned above. By riding both sides of Mayflower Street you can get in a great ride.
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