Around the world, miles of rock are missing. Could ‘Snowball Earth’ be the culprit?

When the famed explorer John Wesley Powell bumped, splashed and thrashed his way down the Colorado River in 1869, he discovered one of the most striking geologic features on Earth. Not the Grand Canyon — although that too is a marvel — but a conspicuous boundary between the sunset-colored sediments of the upper walls and the dark, jagged rocks below them.