The Grand Canyon cuts about a mile deep, exposing rocks as old as 1.7 billion years old at the riverbed. The rim is about 250 million years old (about as old as the riverbed at Canyonlands). The most visible geologic form is the Tonto platform, which is the deceptively flat area of greenish Bright Angel Shale just above the short cliffs of Tapeats Sandstone. The Tonto Platform was deposited during the Cambrian Explosion, when life rapidly developed many forms; trilobites come from this era. Below the platform is the Vishnu group, where life was mostly bacteria.