PIRACY

"Gravel capped wind gap upstream of the bedrock high"

  After a drainage is captured, a drainage divide develops between the uppermost reaches along the paleo-flow direction and the captured drainage.  Because the drainage used to follow the paleo-flow direction and now doesn't, this type of a drainage divide is termed a wind gap.  Gravels usually outcrop along this wind gap, indicating the captured drainage used to flow along the paleo-flow direction's path.  These gravel capped wind gaps represent one of the best pieces of evidence for the piracy mechanism.  A small tributary along South Mountain (near Phoenix, AZ) is depicted in the upper image.  The transverse drainage developed through piracy and a well preserved gravel-capped wind gap is present immediately upstream of the transverse gorge.  The gravels are preserved in a  desert pavement along the wind gap and shown in the lower image (pencil for scale).