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More cliffs across the river, showing the desert varnish, and where slabs have broken off, leaving a clean surface behind. Desert varnish is that black layer, it's made by salts and minerals that used to be the 'cement' that held the grains of sand together as rock (hence the name sandstone) being dissolved by water, then drying on the surface. When these rocks break off, if they fall more than about 60 feet, they reach terminal velocity, and when they hit, it knocks the varnish off, because it is a very thin and fragile layer. Since those cliffs over there are 300 feet or more high, it's tough finding a fallen rock with varnish on it.