The Arched Bridge Conservation Area lies just over the state line, in Dunstable, Mass. The area is named for an old railroad bridge crossing Salmon Brook, a spectacular example of 19th-century engineering.

(Read on to the end for directions to the Treasure Chest)….

Locally-quarried granite blocks were laid without mortar in an arch spanning the brook, and the bridge carried passengers and freight trains on the Nashua Acton and Boston line from 1876 to 1926, according to the excellent railroad history site, Nashua City Station. Development has obliterated all trace of the railroad north of Ridge Road, but the line once continued past Bishop Guertin High School and on into town.

That same stretch of Salmon Brook also is a joy to paddle; I wrote about that here. The railroad path would make a nice return route to the put-in, too.

The trailheads for the Arched Bridge area are easy to reach by bicycle from many Nashua neighborhoods, and it’s a great place for a summer evening's trail ride. The area includes an abandoned stone and gravel pit, and to my eye, the trails are best enjoyed by mountain bike.

Of course, one could hike, run, or (in season) ski or snowshoe there, too. Some of the trails have been gnawed up and widened by ATVs and dirt bikes, but there's also some tasty little single-track along the brook, if you look hard.

HOW TO GET THERE:

There are two ways into the area. First, there is a small footpath off the eastern side of Pinebrook Road in Nashua. It allows public access to the railroad bed, but it runs close by homes, so please be courteous! Pinebrook Road runs to the south off Ridge Road, just east of Salmon Brook.

Alternatively, take Middle Dunstable Road south off Ridge Road, follow it around the bends, and look for a miniscule parking area on the right, next to a dirt road with sign for Arch Bridge Conservation Area. The parking area looks like an abbreviated driveway, and would hold two or three cars, tops… so consider going by bike!

The path from Middle Dunstable Road straddles the state line, and leads straight to the old rail bed.

You may also want to visit the website of the Dunstable Rural Land Trust, a public-private partnership that has protected several hundred acres of undeveloped land in that town. Among their holdings is the 350-acres DRLT Wildlife Preserve, an area between the New Hampshire border, and Main and Fletcher Streets in Dunstable, that also makes for nice and mostly easy off-road bicycling, and includes a lovely pond. You can get there from Main Dunstable Road in Nashua; just look for the area where people are always parked along the south (left, heading toward Hollis) side of the road, out past Gregg Road.

Directions for the Treasure Hunt:

Follow the old Nashua Acton and Boston railroad bed over the stone arched bridge, heading south into Dunstable. Take your first, second or third right off the railbed onto a trail leading into a meadow area with a rocky hill off to the west, the remains of an old stone and gravel pit. Take a right again, and follow the trail around the northern edge of the meadow, and head west up a short, rather steep rise. Keep following the trail past a fire pit, bearing to your right up a gentle climb to a wide gap in the trees, going into a second meadow area. Take a left there, and look for a great cleft in the rocks in the hillside to your left, clearly visible from the trail.

The treasure chest lies hidden within, lodged under one of the larger rocks, in plain sight and easy reach . . . once you’re up close.

Once you’ve grabbed a card, consider following that same trail just for fun: there’s some nice riding out that way. Otherwise, while seeking the treasure, if you pass into the woods beyond the meadows, you’ve gone too far.

The distance from either of the two trailheads to the Treasure Chest is less than one mile, and the trails up to that point are novice (mountain biking) level; a child could ride them, though the short, steeper section might call for a walk.

Here's a photo of the bridge, by Telegraph photo editor Don Himsel:

Don also shot me, riding in the meadow: