Today it is a cliff-former that is thick. Bright Angel Shale averages 515 million years old and is made of mudstone-derived shale that is interbedded with small sections of sandstone and shaly limestone with a few thin beds of dolomite. It was mostly deposited as mud just offshore and contains brachiopod, trilobite, and worm fossils (see 3b in figure 1). The color of this formation is mostly various shades of green with some brownish-tan to gray parts. It is a slope-former and is thick. Glauconite is responsible for the green coloration of the Bright Angel. Muav Limestone averages 505 million years old and is made of gray, thin-bedded limestone that was deposited farther offshore from calcium carbonate precipitates (see 3c in figure 1). The western part of the canyon has a much thicker sequence of Muav than the eastern part. The Muav is a cliff-former, thick. These three formations were laid down over a period of 30 million years from early-to-middle Cambrian time. Trilobites followed by brachiopods are the most commonly reported fossils in this group but well-preserved fossils are relatively rare. We know that the shoreline was transgressing (advancing onto land) because finer grade material was deposited on top of coarser-grained sediment. Today, the Tonto Group makes up the Tonto Platform seen above and following the Colorado River; the Tapeats Sandstone and Muav Limestone form the platform's cliffs and the Bright Angel Shale forms its slopes. Unlike the Proterozoic units below it, the Tonto Group's beds basically lie in their original horizontal position. The Bright Angel Shale in the group forms an aquiclude (barrier to groundwater seeping down), and thus collects and directs water through the overlying Muav Limestone to feed springs in the Inner Gorge. The next two periods of geologic history, the Ordovician and the Silurian, are missing from the Grand Canyon sequence. Geologists do not know if sediments were deposited in these periods and were later removed by erosion or if they were never deposited in the first place. Either way, this break in the geologic history of the area spans about 65 million years. A type of unconformity called a disconformity was formed. Disconformities show erosional features such as valleys, hills and cliffs that are later covered by younger sediments.