It wasn' t hard to visualize their flood plain origin. Look closely at the massive, sheer cliffs. Focus on the base of the wall below. This is crossbedding. Where crossbedding occurs on a large scale like this, it means that here once drifted the windswept dunes of an ancient desert.
Sediment becomes rock when it is buried and compacted by huge overlying loads of other sediment.
Individual sand or clay particles are cemented together by minerals in seeping ground water. As ages pass, the cement of the ancient rock is dissolved by weak acids in rainwater. Small cracks in the rock are widened by frost and plant roots. The rock washes away in chunks and particles. This is what geologists call weathering, part of the larger process of erosion.