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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
Bed Shawangunk Formation
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
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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