STOP 7.  Larger granitic layers crosscutting foliation in the Wissahickon Schist.

    In the rock face you can see the light granite layers and the darker layers of schist and quartzite of the Wissahickon Formation.  At first glance, you might think that the granitic layers are folded into or around layering in the Wissahickon, but on closer inspection you can see that layers of the schist are broken by the granite.  This crosscutting relationship, the granite cuts across the schsist layers, is evidence that the granite is younger than the metamorphism and deformation that have created the layers in the schist.  Because it is impossible to break something that is not there, the granite magma could not intrude into the schist layers and break across them in some places if they were not already there.
    Before geologists were able to use radioactive elements and their constant rate of decay to date rocks, we only were able to tell the relative age of the rocks we found, that is which rock is older and which is younger.  In this outcrop the schist is older and the granite is younger, but that doesn't tell us how old they are or how much older the schist is.  Recent studies done by students and faculty at Bryn Mawr College have dated the peak of metamorphism in similar rocks near Delaware at = 430 million years ago.  Small granitic bodies in the area have been assigned ages of about 350 million years.  However, the age of the granite was determined by methods that we now think may give misleading ages.  Detailed age determinations of the rocks of this area still needs to be done.
 
 
 
 

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