Witte Hi Pt S P Geology

7 they are fractures but unlike faults they show no

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Unformatted text preview: and they commonly form in parallel or subparallel arrays called joint sets. Several different sets may occur in large areas, and they record the regional stresses that caused the fracturing. Ground water moves chiefly through fractures such as joints, so joint analysis helps us to understand ground-water flow. Fine-grained rocks such as shale and slate split along closely spaced parallel planes called cleavage. Cleavage forms under compressive pressure in rocks enriched in clay minerals. Fine8 join t bedding join t join t Ss (Shawangunk Formation) measuring stick (1 unit = 1 foot) Figure 7. Photograph of subvertical joints in a south-facing outcrop of the Shawangunk Formation. Bedding is dipping slightly to the east (right). grained rocks develop cleavage more readily than coarse-grained rocks because they contain more clay. This allows the rock to be split into flat products like chalk boards. The Martinsburg shales exhibit the best-formed cleavage in this region and they have been used for roofing slate and paving stone. Slate is shale in which the cleavage is highly developed. Therefore, the “shales” of the Martinsburg are better termed slates. Siltstones and sandstones in the Martinsburg also exhibit cleavage, but it is generally not as extensive and less closely spaced (fig. 8). Poorly developed cleavage occurs locally in the Bloomsburg Red Beds and the Shawangunk Formation. 9 Om h (Martinsburg Formation, High Point Member) ge ava cle ve in 1 inch in diameter bedding Figure 8. Photograph of cleavage in the slatey member of the Martinsburg Formation looking south (east to the left). Very closely spaced cleavage surfaces, almost paper thin, can be seen dipping to the east. Subvertical veins, formerly open fractures, are filled by quartz, a whiteish mineral. Tectonic History Kittatinny Mountain is part of the Appalachians, a long mountain chain that stretches from Alabama to Newfoundland. Its history includes three periods of continental stretching and collision that all involved folding and faulting. In north...
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