During the Paleozoic era, the Cambrian Explosion occurred, bringing warm temperatures and invertebrates to the area. The Grand Canyon area was near the equator at this point, and oceans began returning. The shoreline moved east and deposited more sediments. These sediments make up the Tonto Group. Tapeats Sandstone is the oldest of the group, made up of coarse sand and conglomerate, speckled with trails of trilobites and brachiopods. The Tapeats Sandstone is fairly erosion resistant, leading it to form the Tonto Platform, where the more erosion susceptible layers of Bright Angel Shale and Muav Limestone rest. The Great Unconformity is responsible for the contact between the Vishnu Basement Rocks and the Tonto Group, completely eroding traces of the Grand Canyon Supergroup in places. The Bright Angel Shale is next and is mainly green and purple in color. Siltstone and shale form slopes on the canyon walls. In places, it intertwines with the Muav Limestone, the youngest member of the Tonto Group. The Muav Limestone is greyish-brown to orange limestone mixed with mudstone. In some areas it is covered by the Devonian Temple Butte Limestone, but most places it is covered by the Mississippian Redwall Limestone. This formation has had little faulting and remains mostly horizontal. The Bright Angel Shale has a barrier to seeping groundwater (an aquiclude) that makes it run through the Muav Limestone, forming several springs in the Inner Gorge.