STOP 8:  A recent fault

    The rocks in this outcrop have been broken by recent faulting, probably related to the uplift of Chestnut Hill.  If you examine the layering in the rocks in the photo, you can see that they don't seem to match. In the foreground the layers are curving to the right while those in the background bend to the left.  The simplest explanation of the mismatch is that there is a break between the outcrops and that the positions of the rocks has shifted along the break or fault.
     The hill of Chestnut Hill is created by a fault that has a long history of movement.  It represents the approximate location where rocks foreign to North America were attached to the North American continent during a series of plate collisions between 450 and 300 million years ago.  The Wissahickon Schists are among those alien rocks.  Towards the end of those collisions as Africa and South America "rammed" into the southeastern edge of North America, the rocks in this area were squeezed out to the east along this same fault.  More recently the fault has been active lifting the area of Chestnut Hill as sediments  deposited off the Jersey shore add weight to the edge of the continent.  As the edge sinks in response to the weight,  Chestnut Hill is lifted like the other end of a seesaw.
 
 
  

RETURN