Slate deposits of Pennsylvania

The slates of Pennsylvania are in the eastern part of the state, in Northampton and Lehigh County within the 'Valley and Ridge Province'. The most important slate quarries stretch from Blue Mountains between Bangor, Pen Argyl via Windgap to Slatington.

The slates are within the Middle- to Upper Ordovician 'Martinsburg Formation' which can be divided into three units - Bushkill, Ramseyburg, Pen Argyl - and reach a total thickness up to 9,843 ft.

The Middle-Ordovician 'Bushkill Unit' consist of fine laminated slates and are named 'lower hard slates' by miners. This is overlaid by the Middle Ordovician 'Ramseyburg Unit', showing a slate - greywacke interbedding.
Up to the top the Middle- to Upper Ordovician 'Pen Argyl Unit' follows and is characterized by fine laminated slates with rhythmically intercalated carbonatic slates, siltstones and fine- to medium coarse greywackes. This 'Pen Argyl Unit' is called 'soft slates' by miners.

The Martinsburg Basin is a depression within a Cambrian to Middle-Ordovician platform which is filled by a northerly trending submarine delta fan complex (Wright et al., 1979). The greywackes show typical turbidity phenomena such as gradded, planar or convolute bedding (Epstein & Lyttle, 1994).

The Taconic Phase lead to folding and thrusting of the Early Paleozoic units in eastern Pennsylvania, although the 'Martinsburg Formation' was only slightly folded during this orogenic phase (Epstein & Lyttle, 1994). The following 'Alleghian Phase' overprinted existing structures and is characterized by another folding (upright to overturned folds), the origin of a cleavage, fault systems and imbricate thrusting. Bedding and Cleavage encompass an angle and sometimes a crenulation foliation is developed.
The colours of the slates vary between grey, grey-black and blue-black. The 'soft slates' are used for the production of roofing slate and slate powder and the 'lower hard slates' for roofing and flooring.

Today, two slate quarries are in operation: One at Pen Argyl and one north of Slatedale

Pictures

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  Fig. 1 Abandoned slate quarry northeast of Bangor

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  Fig. 2 Derricks and carriers to hoist slate blocks in the slate quarry at Pen Argyl.

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  Fig. 3 Derricks and carriers Kabelanlagen to hoist slate blocks in the slate quarry and quarry men at Pen Argyl. One can consider this quarry as an open chamber and the actual depth is 500 feet.

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  Fig. 4 Classical transversal cleavage with an obvious striation (ribbons, Pen Argyl).

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  Fig. 5 Classical transversal cleavage of the "Chapman slate" which is know for its distinct striation (ribbons) which is unique for Pennsylvanian slate (Chapman).

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  Fig. 6 Slate quarry north of Slatedale in Lehigh County (west of Slatington)

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  Fig. 7 Fold in alate quarry north of Slatedale in Lehigh County (west of Slatington)

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  Fig. 8 Efflorescence is a typical phenomenon of slate which were quarried in the "soft slate" unit (Slatington)

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  Fig. 9 Slate of the "soft slate" unit get a dull appearance and signs of delamination can be frequently seen (Danielsville)

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  Fig. 10 Typical brownish colouring of slate of the "soft slate" unit (Bangor)