STOP 3a:  Eroding and depositing streams in the valley of Wissahickon Creek.

    Just beyond the outcrop you will come to a small creek.  If you look uphill, you will see the creek forms a V-shaped valley with the sides of the valley sloping to the edges of the stream.  If you turn and look toward the Wissahickon Creek, the V-shaped vallye disappears and instead, one can see sediments carried by the stream, rocks, dirt, leaves, and tree trunks, spread over a broad fan-shaped area.  The change from eroding to depositing stream reflects the change in slope.  Uphill of the trail, the stream falls rapidly down a relatively steep slope,  In this area it picks up sediments and moves them downstream, especially in times of high flow after a rain.  Near the Wissahickon, the slope is less steep.  The stream slows in this area and deposits material.  The larger rocks and logs cause the stream to change course and spread across a wider area.  The sediments form an alluvial fan.  Here it is only a small feature, but in the West fans can be miles across and be hundreds of feet thick.
   
 
 
  

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