Prob = pitch-pole fence n.
1864 Beckwith Winthrops 38 cNY, Naturally the most interesting feature of the landscape [in VA], was the darkies. . . Here a number sat idly sunning on a back-bone fence. 1870 Peterson’s Mag. 57.223 NEng, Each farm-house turned into a palace of jasper, and the back-bone fences around pastures that bore two rocks to every blade of grass, were built of pearl, and inclosed glimpses of the lost Eden. 1939 LANE Map 117, 1 inf, cwCT, We had rail f[ences], backbone f[ences], and post-and-rail f[ences].