Witte Hi Pt S P Geology

These layers are as much as 6 feet thick and make up

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Unformatted text preview: o rip up and erode some underlying sediment. The upper surface of the Martinsburg Formation is marked by the Taconic Unconformity, a period of 20 to 30 million years during which the upper part of the formation was uplifted and stripped by erosion. The Shawangunk Formation was deposited on this surface; a slight angular difference in bedding orientation between the two formations defines the unconformity (fig. 4). ity B nc cu ing ed d oni Tac orm onf ding Bed Shawangunk Formation Hammer for scale Martinsburg Formation Figure 4. Photograph of the Taconic Unconformity, exposed during the construction of Interstate 84, just north of the Park in New York. View is toward north. Photo taken by Jack Epstein, U.S. Geological Survey. The Shawangunk Formation is of Middle Silurian age (approximately 425 million years ago), about 1,400 feet thick and consists of interlayered pebble conglomerate, quartzite, and some shale. The conglomerate mostly contains rounded pebbles of quartz with lesser chert and shale in a matrix of quartz sandstone. Sediment 5 grain size becomes finer upward over several layers, forming repeating sequences of conglomerate that change from sandstone, to siltstone and finally to shale. This grain-size composition of the Shawangunk and its sedimentary structures (fig. 5) show that these materials were deposited by braided streams in alluvial fans. A braided stream flows in several dividing and reuniting channels that resemble a hair braid. Alluvial fans are large cone-shaped Ss (Shawangunk Formation) bedding bed ss- cro bedding measuring stick (1 unit = 1 foot) Figure 5. Photograph of cross-bedding in the Shawangunk Formation at Stop 1. View is toward north. The regional bedding surfaces are outlined and labelled. The cross-bed is clearly outlined in the rock by the more steeply inclined surfaces. deposits laid down by rivers flowing out from mountainous areas onto low level plains. Most of the Shawangunk sediments were carried far out on the plain and away from the mountains. The cross-bed orientations in the Sh...
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