The Pennsylvania “Slate Belt,” an area of only twenty-two square miles, lies approximately fifty miles to the northwest of Philadelphia and just south of Blue (Kittanning) Mountain between the Delaware and Lehigh rivers. The first quarries opened in the 1830s, but significant growth followed in the first decade of the twentieth century when the Lehigh Valley accounted for approximately half of the slate produced in the United States, eventually becoming the greatest slate-producing region in the world. During World War I, many of the slate firms closed to release men for other essential war work, especially in the Bethlehem Steel plant nearby. Most of the quarries never reopened after the war, as modern synthetic materials such as asphalt composites and plastics proved less expensive and easier to use and required less skilled labor to fabricate and install.
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