A new study led by researchers at the University of Southampton proposes a colossal escarpment that formed around 800 million years ago as supercontinent Rodinia began breaking apart. This ancient cliff, named the Great Escarpment of Laurentia, may have retreated inland over time, eroding up to 8 kilometers of rock and explaining the Great Unconformity - a billion-year geological gap preserved in the Grand Canyon today. The study, published in Geology, combines plate-tectonic reconstructions with landscape evolution models to suggest that this escarpment extended across much of North America, from modern Arizona to Illinois. The retreating cliff could have allowed glaciation during the Snowball Earth period to remove additional material, contributing to the Great Unconformity. The study also identifies Death Valley as part of a lower-lying basin associated with the ancient escarpment, and provides insights into how modern escarpments in South Africa and Brazil may help reconstruct the long-lost landscape along the margin of Laurentia.