A geologic cross-section of the rock units in the lower Grand Canyon. A horizontal scale shows the field of view is about 100 miles across (160 km). A vertical scale shows the thickness of the units depicted is about 1500 feet thick in total (~450 meters). Precambrian rock units are shown at the bottom of the diagram, with wispy swirls indicating they are deformed. Then, going upward, there is an unconformity surface, followed by the Tapeats Sandstone. The Tapeats Sandstone is overlain by the Bright Angel Shale, which is overlain by the Muav Limestone. The Tapeats is a little thicker in the east (the right side of the image). The Muav is much thicker to the west (left side of image). The western (left) Tapeats and Bright Angel show a blue dashed line indicating the "zone" (presence) of a trilobite of Early Cambrian age. The upper Bright Angel in the west (left) and the lower Bright Angel in the east (right) show a dashed red line indicating the "zone" of a trilobite of Middle Cambrian age. (This indicates that in the west, most of the Bright Angel Shale is older than Middle Cambrian, but in the east, most of the