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The Devonian Story of the Hudson Valley click |
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| An angular unconformity in Paleozoic rocks. The Law of Original Horizontality states that, with but few exceptions, layers of rock are deposited in a flat-lying position. Therefore, when a geologist sees rock units that are deformed in some way (folded, or, in this case, tilted), he or she can infer that something must have disturbed them after they were deposited. Here, rocks of Ordovician age were tilted, then eroded, to create somethig known as an "angular unconformity". Then, rocks of Silurian age were deposited on top of this unconformity. Finally, the entire rock package was tipped to its present position. | |
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Contact between the Coeymans and
Kalkberg Formations, both of the Helderberg Group, Devonian period.
The contact is approximately at this student's shoulder. The base
of the Kalkberg is recognized by the appearance of chert, seen here
as dark, knobby bands. The first band is noticable at the very top
of this student's head. Again, the dip of the rocks indicates that
something must have occurred to tilt the rocks from their original
flat-lying position. This "something" is the Acadian orogeny,
a mountain-building event in the Devonian period.
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| A fault seperating the older Manlius Formation (above) and the younger Kalkberg Formation (below). The only way for older rocks to be positioned above younger rocks is to force them into that position with a thrust fault. In this case, the older rocks of the Manlius Formation have been thrust up and over the younger Kalkberg Formation. | |
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Fault breccia from the fault of the preceeding
photograph. These angular fragments along the shear zone result
from the crushing, shattering and shearing of rocks during the movement
associated with faulting. Fluids rich in dissolved calcite percolated
through the fracture zone, precipitating large crystals of calcite,
the white mineral you see in this photo.
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Slickensides, beautifully preserved atop a bedding
plane in the Manlius Formation of the Helderberg Group. Slickensides
are polished and striated surfaces that form along a fault plane,
and can be very instrumental in determining the direction of movement.
Often, as rocks are deformed, zones of greatest weakness occur along
bedding planes, where it is easy for rocks to slip past one another,
usually for very small distances. This is the case in this example.
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