slickensides

slickensides are parallel striations on rock surfaces produced by relative motion across opposite sides of fault planes. they can give important information about the sense of shear and direction of motion across the fault plane. these slickenlines indicate a top-to-the-west, thrust sense of shear along the interface of a coal bed with a sandstone bed. the coal (with its low internal strength and high graphite content) acts as a weak layer that allows the overlying shale to 'glide' across the sandstone.



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