Haystack Mountain is a modest 1,871-foot summit at Solomon Gap in Mountain Top, Pennsylvania, but its significance lies in the geography it helps define. Together with Penobscot Mountain and the gap, it marks a drainage divide between the Lehigh and Delaware basins and the Wyoming and Susquehanna valleys. In a region where ridges often blocked travel, Solomon Gap became one of the few practical corridors between the Wyoming and Lehigh valleys, later carrying railroads and major roads. In the 1830s it drew railroad planners seeking access to northern anthracite fields, and later competing lines crossed its slopes, including a route used by the Black Diamond Express.