Imagine that you come upon a gently sloping river valley bordered by nearly vertical rock cliffs. What explains this landscape? Imagine that you come upon a gently sloping river valley bordered by nearly vertical rock cliffs. What explains this landscape? The valley is composed of highly erodible sandstone and the cliffs of resistant igneous rock. Valleys of this type form where the bedrock is composed of basalt. The valley may have been the epicenter of a large earthquake. The near-vertical cliffs are made of highly erodible sandstone. This is a landscape of folding rather than faulting.