Just about everything in purple ("access trail" or double track) on the trailforks map sets are gonna be a dirt road of some sort (ie "gravel") in usually a passable condition by a gravel bike. Arcadia/Beach Pond/Yawgoog Scout Reservation and across the border into CT in Pachaug State Forest are an absolute rats nest of gravel trails, lots of fun to make connections and loops. Carolina Management Area down into Burlingame has a pretty good network of gravel as well. The rail trails (Washington Secondary/South County/East Bay) show up purple but are paved. The one exception I can think of is the Trestle trail- it's still partially gravel at the western edge where it meets CT. There's some roads in CT down by Yawgoog (Sand Hill/Denison Hill) that are partially paved and partially gravel but show up as gravel for some reason.

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A favorite is a lollipop that departs the exit 1 park and ride, heads north to Piney and then either climbs up Denison Hill or detours over towards Tom Harvey before rejoining behind the Scout camp, close to the RICONN marker. You return to the park and ride via North Road and one absolute ripping fast descent. A longer version heads north into Green Falls Pond via the various cart and access roads, spends a bit of time on 138 and 165 (turning back into the woods just before Beach Pond on the Tippicansett trail). You then working your way thru Yawgoog on the Galkin trail (its a bit cryptic in places if we're being honest but look for the openings in the mountain laurel and the old mtb trail bridges from the mid 90's) and back to the car via North road.