Paired and unpaired terraces: Terraces of the same elevation on opposite sides of either a stream or river are called "paired terraces". They occur when it downcuts evenly on both sides and terraces on one side of the river correspond in height with those on the other side. Paired terraces are caused by river rejuvenation. "Unpaired terraces" occur when either a stream or river encounters material on one side that resists erosion, leaving a single terrace with no corresponding terrace on the resistant side.