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The Taconic Orogeny of the
Middle Hudson Valley
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to view the location map
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Pillow basalts at Stark's
Knob, just north of Saratoga, famous for both geologic and
historic reasons (this hill figured prominently in the Battle
of Saratoga during the Revolutionary War.) Many pillow basalts
form in deep waters where sea-floor spreading occurs. Lava
from melted oceanic crust hits the water and cools rapidly,
forming the "pillows". The story of these pillow
basalts may not be so simple: some have made the interpretation
of a shallow water pillow basalt formed during the time of
the Ordovician Taconic Orogeny which was then thrust into
this position from the east. Pencil at the
center of the picture for scale. |
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This is the Giddings Brook Thrust at Bald
Mountain, near Saratoga. Although this picture is a bit
obscured by the vegetation, here we see Cambrian slates
over-riding Ordovician limestone. This thrusting is a result
of the Taconic Orogeny during the Ordovician period. Though
not seen in this picture, this limestone is also sitting
atop younger rocks. These rocks were all caught beneath
the overthrust and "bulldozed" in from the east.
Pieces of rock, some as large as several kilometers in diameter,
broke off from the main "allochthon" (defined
as "a mass of rock moved a great distance from its
place of origin). For scale, this outcrop
is about 10 meters (~30 feet) wide.
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This brown rock is the Austin Glen Formation
(late Ordovician), which is the last rock unit we see deposited
in the foreland basin before this basin disappeared in front
of the oncoming allochthon.The white rock is an early Ordovician
limestone clast - material from an underlying formation.
As the allochthon was moving from the east, material was
being shed into the foreland basin to the west. This is
an example of an older rock unit being thrust up and forming
mountains, which were in turn eroding and depositing into
the foreland basin.
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This rock is known locally as "Dinosaur
Leather", because of its apparent similarity to dinosaur
skin. In fact, this is the Austin Glen Formation, an Ordovician
siltstone. The "bumps" on this surface are actually
"flute casts". These form as a result of turbulent
sediment-laden current scouring a muddy bottom. This indicates
swift currents, probably turbidites, as sediment was being
shed off of the newly-forming mountains to the east. This
rock has been tilted so that what we are seeing is the underside
of a bedding plane; these rocks are nearly vertical. Hammer
at the bottom of the photo for scale.
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