Witte Hi Pt S P Geology

Structure of the rocks compressional or pushing

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Unformatted text preview: form by compressive force, similar to pushing two ends of a piece of paper together. The arches and troughs that form are called anticlines and synclines respectively. Fold magnitude varies in each formation, but in the park it is on the order of several hundred feet (fig. 6). Generally, folds cannot be seen at an individual bedrock outcrop. One needs to walk across the rock and note the slow change in the dip of the beds from southeast to northwest. A fault is a fracture or group of fractures in rock where one side has moved past the other. Such breaks form when rock is squeezed or pulled apart. An offset of sedimentary layering marks the location of a fault and helps to determine the rock’s movement direction. Faults that parallel layering are harder to discern. A fault commonly exhibits linear grooves that show the direction of 7 EXPLANATION Ss Omh High Point Monument Bloomsburg Red Beds Shawangunk Formation Martinsburg Fm., High Point Member Taconic unconformity bedding anticline 2000 FEET 1000 SEA LEVEL -1000 Sb Lake Marcia syncline Sb Ss Omh anticline syncline Ss land surface 1000 Ss Omh Omh SEA LEVEL -1000 1 mile -2000 2000 FEET Sb -2000 Figure 6. Regional crosssection through Lake Marcia from the northwest (left) to the southeast (right). The view, taken from the Lake Marcia beach, faces north toward the monument. motion. These grooves form where bumps on the fault surface slide past and gouge the opposing rock surface. Quartz veins are also common in faults and their alignment is similar to that of the grooves described above. Faults cut the three rock formations in the park; all are small, have minor offsets, and are no longer active. They are most common in the brittle rocks of the Shawangunk Formation. Joints are the most common structural feature in rocks (fig. 7). They are fractures, but unlike faults, they show no evidence of movement parallel to their common surface. Joints form where pulling or pushing forces caused the rock to break. Joints may cut across a whole outcrop or only an individual bed,...
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