What it was
A boat slips between the high walls of the Grand Canyon. On the walls, old rock meets younger layers at a sharp line. John Wesley Powell saw breaks like these during his trip down the Colorado River.
Powell was a geologist, someone who reads rocks to work out their past. He recognized missing time where the layers met. That kind of gap is an unconformity (un-kon-for-mih-tee): a stretch of Earth's rock record that is absent from the stack.
When and where
Powell's river trip passed through the canyon in 1869. He described the rock contacts in a report in 1875. The chapter follows those clues into the Great Unconformity, a set of gaps found in the canyon and other places.
Put your hands on either side of some contacts and more than a billion years can lie between them. Other gaps are shorter. In one place, tilted beds sit beneath flat ones; elsewhere, younger layers rest on old rock full of crystals.
How we know
The line is easy to see. Dating it takes more work. Scientists read clocks inside zircon, a tough crystal whose atoms change at a known rate. They date rocks on both sides of the gap to measure how much time is missing there.
Other tests tell when buried rock cooled as it rose toward the surface. That can help trace erosion, the wearing away of rock above it. A cooling date tells a different part of the story from the day a crystal first grew.
Still a mystery
Scientists still argue about what removed the missing layers. Huge glaciers may have helped in some places; rising land and ordinary weather did work elsewhere. The gaps formed at different times. Powell could see the contact, but the full story lies in the rocks on either side.