Boulder fields (Rock Rivers) are common in Pennsylvania where slopes
facilitate the downslope movement of large blocks weathered from bedrock
by freeze-thaw action. Probably small amounts of water beneath the rocks
freezes in winter lifting the rocks perpendicular to ground surface. When
the ice melts, the rock falls straight down. This results in a small downslope
movement that is multiplied by each freezing and melting. Water beneath
the Hickory Run Boulder Field can be seen in this photo. Additional
photos at http://www.pvbears.org/compclub/boulder.htm .
