Grand Canyon, Great Unconformity (IMAGE) University of Maryland Caption This view of the Grand Canyon shows the Great Unconformity, a boundary where nearly a billion years' worth of sedimentary deposits is missing from the geologic record. The boundary can be seen at roughly the middle of this image, separating the older, lumpy and angular rocks below from the younger horizontal layers above. New research suggests that the missing sediments, likely scrubbed away by glaciers during the global "snowball Earth" that ended roughly 635 million years ago, washed away to the oceans, where they lubricated subduction faults and kick-started the modern age of plate tectonics. Credit USGS/Alex Demas Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.