This is a thick, red sandstone layer. Sand blows up the back side of the dune, an tumbles down the front face. These record succession of shallow sea, coastal plain, stream deposits, lake deposits, sand dunes, then more streams, and shallow sea. Most river erosion occurs when the river is in flood. Flash floods occur when there's so much rainfall that water can't sink into ground.
Excess water becomes runoff, and spills into the stream, so water rises.
Fast, turbulent flow can lift and carry boulders. In the upper part of the Park, have a sweeping landscape of exposed rock with cross-bedding exposed on surfaces. Cedar Breaks Park - 25 miles north of Zion.