Where the Mountains Meet the Sea
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It is axiomatic, and I have said it many times here, that you almost can't find an unscenic road in northern New England. This is where, after all, the Appalachian Mountains which have been running north just inland finally run right up against the coast, symbolized by Cadillac Mountain in Acadia National Park. I and others have also written previously about some of the highlights of New England and I would urge you to read through those discussions to get an idea of the wealth of options available to you. For now what I'm going to do is to suggest a route that will let you see some of the best the coast has to offer, then takes you into the mountains for a while, and finally returns you to Portland by way of Auburn.
Portland itself is a great place to spend a day or two exploring the waterfront, the Eastern Promenade, and Portland Head Light. Heading 'Down East' from there, a short run up I-295 will bring you to Freeport where a stop at L.L. Bean's (open 24/7) is in order before switching over to US-1 to follow the coast, more or less. I say more or less because the Maine coast along here consists of a number of peninsulas that jut out into the sea. My favorite is the one south of Damariscotta which you can make a loop of by using ME-129/ME-130/ME-32. ME-129 goes down a sub-peninsula to a great little town, Christmas Cove, at the very end of the quintessential rock-ribbed coast. You'd than back track to ME-130 and continue around the main peninsula to Fort William Henry, Colonial Pemaquid, and Pemaquid Point Lighthouse. Return to US-1, head for Rockland, and take the ferry over to Vinalhaven for a day or so. Finish up your coastal leg by heading up to Bar Harbor and Acadia National Park. September will be a good time to visit there. The summer tourists will have mostly left and the evenings will be starting to take on a fall crispness.
Next I'd suggest that you turn inland heading, eventually, for the Presidential Range of the White Mountains in New Hampshire. Taking US-1A up to Bangor (Stephen King country) an then US-2 west across Maine will let you get a good taste of the Maine woods. There aren't a whole lot of covered bridges in Maine, but there are some. The Shelburne Birches line the road as you leave Maine and enter New Hampshire. You might want to take a slight detour south from Gorham NH on NH-16 to Glenn House and drive the Mount Washington Auto Road to the top of that mountain, but if you do, return to Gorham and continue west on US-2 to NH-115 south to US-3 south which becomes co-aligned with I-93 as a parkway through Franconia Notch State Park. Here take the time to hike the Flume Trail before continuing down to Lincoln and turning east on NH-112, the Kancamagus Highway, one of the great scenic roads in northern New England. At the eastern end of this road, at Conway NH, there is a chance to take another short side trip (north this time) on NH-16 to North Conway and its many outlet stores.
Your final leg, then, would be to take US-302/ME-11 through the Maine Lake District (Sebago Lake State Park, Range Ponds State Park) to Auburn and your dinner before heading back down either the Maine Turnpike (I-95) or US-202/US-302 to Portland
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