What could cause a forest of bent trees?
The phenomenon in question is probably related to geotropism.

If the hill soil is "on the move" it will cause the bend on the trees - 


  If the soil in a slope is moving downward, the trees on this slope
  will tip downward.  As the tree continues to try to grow upward, the
  trunk will show a curve. The degree of bending could indicate the rate
  or amount of movement of the soil.




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