Last weekend we were riding bikes between the Anticline overlook, Canyonlands overlook and the Needles Overlook in the Canyon Rims National Recreation area.
Looking just above the Colorado River towards the Islands in the Sky, you can see a white strip creating a rather wide bench. The bench is named the White Rim and the rock unit is named after the bench. The sandstone is a brilliant white (hence the name) quartzose. In early Permian times, this region was a great inland sea of sand.
When I talk with teachers and students they almost universally understand that sediment was originally deposited horizontally. Most can at least intellectually understand the geologic time scale. What they don't seem to understand is that the rock unit does not exist in all places. Most of my students expect the White Rim, Wingate, Chinle etc to be found in all areas of the Colorado Plateau.
What seems to happen though in the translation from my field lecture (pointing, waving and talking) to their brains is that they believe that each rock unit is of equal depth for its entirety. It is only through much diagramming of sea level change and beach strands moving all over the state for me to get the bulb to light up. In this case, the White Rim's showcase is here on the Rim itself. As you travel north and east the rock unit itself thins out to nothing.
For a more in depth look at the geology of the White Rim sandstone visit Geotripper.