Sediment settles out of water in layers, with the biggest and heaviest pieces first. Each new layer lands on top of the last, so in an undisturbed stack the bottom layer is the oldest. Geologists read rock layers, called strata, and the fossils in them to put events in order. That gives relative ages, older or younger, not a number of years. This week the crew shakes sand, soil and gravel in a bottle of water and reads a canyon wall.