Slate deposits of Maryland and Pennsylvania (Peach Bottom Slate)

'Peach Bottom Slate' describes the slates of the Peach Bottom area at the border between Pennsylvania (PA) and Maryland (MD). The main localities are Delta (PA) and Cardiff (MD) and there was a striking discrepancy between the importance of mining and trading which can be read in Mathews (1898):

„Unfortunately Maryland has received little credit for its share in the industry although almost all of the productive quarries are situated within its limits. This apparent injustice has arisen from the fact that the shipping point for most quarries and the residence of many of the operators is Delta [...] Delta is so much better known than its Maryland associate, Cardiff, that mail is received through the Delta postoffice by inhabitants living scarcely one hundred yards from the Cardiff office.“

The slates are situated within the 'Peach Bottom Syncline' a narrow ca. 15 mi. structure which stretches from Kings Bridge, Lancaster County (PA) via Pylesville, Harford County (MD) to the 'Arvonia slate' in Virginia and belongs to the 'Piedmont Province'. This syncline consist of Early Paleozoic rock units of uncertain exact age. Depending on the folding style of the syncline, the thickness of the 'Peach Bottom Slate' can vary between 492-984 ft.

The almost vertical fracture cleavage is frequently cut by a younger foliation, resulting in a wrinkled surface and thus the slate cannot be used for roofing slates. In quarries where slate was mined the first fracture cleavage dominates. This slates are related with a homogeneous siltic sedimentary pattern, resulting in a homogeneous appearance of the slate (Southwick, 1969). The slates are generally very hard with a blue-black colour and a shiny surface. Southwick mentioned as main components quartz, muscovite, chloride with a strong alignment of the phyllosilicates.

Besides the 'Peach Bottom Slate', in Maryland are also slate deposits in Hyattstown, Ijamsville and Linganore (Harford County), although - except for Ijamsville - slate was never seriously mined. Even in Ijamsville, slate mining was hardly carried out during the civil war (1861-1865) and was finally finished in 1870. Mathews (1898) mentioned, that here the slate was not mined opencast but rather subsurface.

Pictures

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  Fig. 1 Cottage of the Welsh slate quarry men. The remaining houses are preserved in original conditions (Coulsontown)

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  Fig. 2 Detail of the slate clock in the Old Line Museum in Delta (Pennsylvania).

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  Fig. 3 The slate quarries are mostly flooded and difficult to access.

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  Fig. 4 Peach Bottom Slate on the roof of the Old Line Museums in Delta (Pennsylvania)

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  Fig. 5 The "Rehoboth Welsh" Church in Delta (Pennsylvania).

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  Fig. 6 Detail of Fig. 5

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  Fig. 7 Rich ornamented tombstones from Peach Bottom Slate on the cemetery of the Slateville Presbyterian Church

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  Fig. 8 Detail of Fig. 7

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  Fig. 9 Roof of the United Methodist Church in Delta

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  Fig. 10 Peach Bottom Slate is supposed to be a hard slate. Its colour varies between black and black-blue.