Put one hand on a canyon wall and the other just above it. The rocks can be more than a billion years apart in age, though your hands sit only inches apart.
An unconformity is a gap in the rock record. John Wesley Powell saw major breaks in the Grand Canyon during his 1869 trip down the Colorado River. Old rock meets much younger layers, with a huge slice of time missing between them.
Scientists date the rock on either side. Zircon crystals carry clocks made from atoms that change at a known rate. The missing rock has no clock left to read.
But some walls give the loss away. Old layers tilt sideways and end at a flat cut, with new layers resting across them. Rock had to build up, tip, and wear away before that top layer formed.
Scientists still argue about what erased so much. Huge glaciers may have helped, while rising land and ordinary weather wore other places down. Tests that track when buried rock cooled help tell those stories apart. The gaps formed at different times, even within the canyon. There was no single known day when the whole world's pages tore out.
