Ancient 'Mega-Escarpment' May Explain Grand Canyon's Missing Billion Years

A colossal cliff that once stretched across ancient North America may have exposed the deep rock now visible at the bottom of the Grand Canyon nearly a billion years before the Colorado River carved the canyon itself, according to a new study led by the University of Southampton in the UK.

The study, published in the journal Geology, proposes that a vast "great escarpment" of steep cliffs, roughly a kilometer high, formed when the supercontinent Rodinia broke apart about 800 million years ago and extended along much of western North America, through what are now Arizona, Utah, Idaho, Wyoming, Colorado, Texas, Oklahoma, Arkansas, Missouri and Illinois. As the escarpment slowly retreated inland over tens of millions of years, the researchers estimate it stripped away as much as 8 kilometers (5 miles) of rock in places, exposing North America's ancient crystalline basement, including the rock now seen at the Grand Canyon.

"The Grand Canyon has a geological record spanning 2 billion years, but more than half of its rock record appears to be missing," said Thomas Gernon, a professor of Earth science at Southampton and the study's lead author. The team, which also included researchers from Germany's GFZ Helmholtz Center for Geosciences and University of Potsdam, and the University of Illinois Urbana-Champaign in the US, combined plate tectonic reconstructions with landscape evolution data and found the Grand Canyon once sat in the same position relative to the ancient continental margin as today's great escarpments in South Africa and Brazil.

Gernon said the findings help explain the Great Unconformity, a gap of more than a billion years in the rock record, and are consistent with evidence that the region lost 5 to 10 kilometers (3 to 6 miles) of rock before the modern canyon formed. The ancient escarpment may also have shaped where rivers flowed and sediment accumulated across the region before the Cambrian explosion, the rapid diversification of complex life.