<P> Subsequent erosion lasting 300 million years stripped much of the exposed sediments and the mountains away . This reduced the very high mountains to small hills a few tens to hundreds of feet (tens of meters) high . Geologist John Wesley Powell called this major gap in the geologic record, which is also seen in other parts of the world, the Great Unconformity . Other sediments may have been added but, if they ever existed, were completely removed by erosion . Such gaps in the geologic record are called unconformities by geologists . The Great Unconformity is one of the best examples of an exposed nonconformity, which is a type of unconformity that has bedded rock units above igneous or metamorphic rocks . </P> <P> In late Precambrian time, extension from a large tectonic plate or smaller plates moving away from Laurentia thinned its continental crust, forming large rift basins that would ultimately fail to split the continent . Eventually, this sunken region of Laurentia was flooded with a shallow seaway that extended from at least present - day Lake Superior to Glacier National Park in Montana to the Grand Canyon and the Uinta Mountains . The resulting Grand Canyon Supergroup of sedimentary units is composed of nine varied geologic formations that were laid down from 1.2 billion and 740 million years ago in this sea . Good exposures of the supergroup can be seen in eastern Grand Canyon in the Inner Gorge and from Desert View, Lipan Point and Moran point . </P> <P> The oldest section of the supergroup is the Unkar Group . It accumulated in a variety of fluvial, deltaic, tidal, nearshore marine, and offshore marine environments . The first formation to be laid down in the Unkar Group was the Bass Formation . Fluvial gravels initially accumulated in shallow river valleys . They later lithified into a basal conglomerate that is known as the Hotauta Member of the Bass Formation . The Bass Formation was deposited in a shallow sea near the coast as a mix of limestone, sandstone, and shale . Diagenesis later altered the bulk of the limestone into dolomite . It is 120 to 340 feet (37 to 100 m) thick and grayish in color . Averaging 1250 million years old, this is the oldest layer exposed in the Grand Canyon that contains fossils--stromatolites . Hakatai Shale is made of thin beds of marginal - marine - derived mudstones, sandstones, and shale that, together, are 445 to 985 feet (136 to 300 m) thick . This formation indicates a short - lived regression (retreat) of the seashore in the area that left mud flats . Today it is very bright orange - red and gives the Red Canyon its name . Shinumo Quartzite is a resistant marine sedimentary quartzite that was eroded to form monadnocks that later became islands in Cambrian time . Those islands withstood wave action long enough to become re-buried by other sediments in the Cambrian Period . Dox Formation is over 3,000 feet (910 m) thick and is made of sandstone with some interbedded shale beds and mudstone that were deposited in fluvial and tidal environments . Ripple marks and other features indicate it was close to the shore . Outcrops of this red to orange formation can be seen in the eastern parts of the canyon . Fossils of stromatolites and algae are found in this layer . At 1070 ± 70 million years old, the Cardenas Basalt is the youngest formation in the Unkar Group . It is made of layers of dark brown basaltic rocks that flowed as lava up to 1,000 feet (300 m) thick . </P> <P> Nankoweap Formation is around 1050 million years old and is not part of a group . This rock unit is made of coarse - grained sandstone, and was deposited in a shallow sea on top of the eroded surface of the Cardenas Basalt . The Nankoweap is only exposed in the eastern part of the canyon . A gap in the geologic record, an unconformity, follows the Nankoweap . </P>

What is the second layer of the grand canyon made of