See quotations.
1851. The pumpkin Pine is generally found on flat land and in ravines; also on abrupt ridges, called horsebacks, where the forest is dense.—John S. Springer, ‘Forest Life,’ p. 41 (N.Y.).
1857. There were singular long ridges, called horsebacks, covered with ferns.—Thoreau, ‘Maine Woods’ (1894), p. 390. (N.E.D.)
