The Grand Canyon itself is a late Cenozoic feature, characteristic of renewed erosion during this time. Vigorous cutting by the snow-fed Colorado River carved the Canyon’s depth. Canyon widening is held in check by the region’s dry climate. The asymmetry between rapid downcutting and slow widening results in the Grand Canyon rather than a more typical broad (and nondescript) river valley. Although violent storms may send flash floods gouging down narrow side canyons, the lack of steady moisture has created a stark landscape of mostly naked rock. Harder, erosion-resistant rocks such as the Coconino Sandstone and the Redwall Limestone have eroded into bold cliffs. Softer layers melt into slopes like the Tonto Platform (Bright Angel Shale) and the Esplanade (Hermit Shale). The oldest, crystalline rocks are chiseled into the craggy cliffs of the Granite Gorges.