The Peach Bottom Slate (PBS) is found within a complex metamorphic terrain in Lancaster and York counties, Pennsylvania, extending into northern Harford County, Maryland. The formation is considered to be Ordovician in age, but its structure is still not completely understood. The earliest known quarrying of the slate goes back to Welshman John J. Roberts in the 1730’s. The first commercial slate quarry in the United States was opened in this area in 1785. In 1843, a large Welsh migration brought experienced miners into the area. With the introduction of the Welsh, a new technique of straight-walled deep surface mining was introduced. Approximately 34 quarries once operated within the PBS on “Slate Ridge.” At the 1850 World Expo in London, England, the PBS was voted the “best building slate in the world.” Uses for the slate included grave markers, shingles, sidewalks, windowsills, fence posts and stove plates. The slate has been used in many federal and state buildings as well as the Biltmore Mansion in Asheville, North Carolina. The final use of the slate in the 1930’s and early 1940’s was roofing granules at the Funkhauser‘s Quarry. Due to it’s higher than normal content of silica and aluminum, the PBS keeps its original color and does not weather. A local church has had a PBS roof on it for 200 years without any leakage.

Today, the heritage of the slate industry is well preserved through dedicated efforts of Delta area residents. The Old Line Museum, privately funded organization, contains a wealth of artifacts, records, pictures of the industry and the world-famous “Slate Clock.” School and civic organizations regularly visit the area. Many of the open pits still exist today, one of which is used for recreational scuba diving. Several residents have learned the art of slate splitting, demonstrating their talents during tours and at the annual fall festival.