The Topsfield Gap Filler Annex (GFA) is located on top of Musquash Mountain in Topsfield, ME.
During the fifties and sixties, several Aircraft Control and Warning (AC & W) "gap filler" sites were constructed along the borders of the United States and across many of the "lower 48" states and Canada. These sites filled in the low altitude gaps between the long-range radar stations.
Sites built before 1958 would have been part of a manual radar defense network. After 1958, new sites would have been part of the Semi-Automatic Ground Enviroment (SAGE) radar network and existing sites would have been upgraded. The last gap filler site in Maine was closed in 1968 and their phase-out continued across the country until approximately 1970.
To reach the Topsfield site from Bangor, take I95 north to Lincoln. Get off at the Route 2/Route 6 exit and head east into town. Stay on route 6 east through Lee and Carroll and cross into Washington County. After you cross into Washington County, it's only a few short miles to East Musquash Lake, which will be on your left. At the far east end of the East Musquash Lake you should see Tower Rd on your right (south). It should be easily identified by the yellow gate at the end and the power lines running up it.
These pictures were taken over the course of two visits in 2009, May and September. When I visited in May, the diesel tank was still on site. In September, nothing was left but the concrete footings. Equipment tracks were fresh, so I assume that it hadn't been gone long. Some photos may be out of chronological order, just to keep a "flow" to the virtual tour of the site.
Special thanks to Bill R for the vintage Topsfield images.