GEOLOGY AND USE OF THE PEACH BOTTOM SLATE, MARYLAND AND PENNSYLVANIA

The Peach Bottom Slate (PBS) lies within the eastern portion of the Piedmont Uplands Section following the northeastern regional strike beginning in Harford County, Maryland extending about 12 miles through York and Lancaster Counties, Pennsylvania. The PBS is a part of a complex metamorphic terraine consisting of schists, graywackes, quartzites and metaconglomerates. Serpentinites are found with these rocks. The age of these rocks is believed to be Lower Paleozoic. The structure of the PBS has been debated for many years. Early geologists such as Jonas and Knopf (1929), Stose and Jonas (1939) and Behre (1950) interpreted it as a syncline. Higgins (1972) suggested an anticline. All of these geologists agreed that a fault existed on the south side of the structure. Later research conducted by Valentino (1990) and others (2003) proposed a shear zone. The most recent investigators, Faill and Smith, II (2010), proposed an anticline bordered by faults on both the northern and southern sides.

Slate in this area was discovered in 1735. It was within the PBS that the first commercial slate quarry in the United States was opened in 1785 by William Docher. In the 1840’s, the Welsh immigrated to the United States. With their knowledge of quarrying slate, the Welsh made the PBS the most sought after roofing material in the country. With its fire-proof resistance, slate was used on roofs to rebuild towns that suffered major fires. Also, many federal and state government buildings in the Mid-Atlantic states used slate. The largest mansion in the country, the Biltmore Mansion in Asheville, North Carolina, has a PBS roof. Other uses for the slate were tombstones, fence posts, window sills, foundations and carvings. During the peak period of the late 19^th and early 20^th centuries, about 27 quarries operated within the PBS. The industry strived until about 1914 when other materials were being introduced as a substitute. The final use of the slate was for shingle granules lasting into the 1960’s. It is amazing that an industry such as this was able to survive for nearly 175 years with an estimated 80% wastage of material. Today, with the dedicated efforts of the residents of Delta, the heritage is being preserved by operating the Old Line Museum and renovating two of the original Welsh cottages in Coulsontown.