Ash Beds and a Boundary Clay: Dating a Dinosaur Dig

Two Weeks in the Badlands

1Each July, a university museum runs a two-week dig in the badlands of the northern Great Plains, and it saves a few places for middle school volunteers. This year Beatriz Holmqvist, who is about to start eighth grade, joined the crew. Volunteers brushed loose rock away from bones, sifted crumbly mudstone for tiny teeth and labeled every find with the layer it came from. They worked only inside the marked quarry areas, wore hats and gloves, and left the sharp digging tools to the adult crew.

2The crew leader, Dr. Lise Marchetti, showed the volunteers a hillside where the layers lie flat and undisturbed. Partway up runs a thin, pale clay layer, only about 1 cm thick. The clay holds an unusually large amount of iridium, a metal that is rare in Earth's crust but more common in some asteroids. A clay layer with the same iridium spike, in the same position just above the youngest non-bird dinosaur fossils, has been found at many sites on several continents, both in rocks that formed on land and in rocks from the deep sea floor.

3The hillside also holds two thin beds of volcanic ash. Ash from an eruption settles over a wide area within days, and labs can measure how long ago the crystals in it formed. Table 1 lists every unit of the hillside from the top down, with the fossils the crew has found in each. Because each ash bed settled in a short time, it gives an age for one level of the hill. The layers between the ash beds can be dated only from where they sit compared with the ash.

4Geologists organize Earth's 4.6-billion-year history into a time scale. Its largest divisions are placed where the fossil record changes sharply in rocks around the world. The Paleozoic Era began about 540 million years ago, the Mesozoic Era began 252 million years ago, and the Cenozoic Era began at the time of the boundary clay and continues today. The long stretch before 540 million years ago is called Precambrian time.

5Back at the museum, Beatriz copied a list of major events from a wall display. It appears as Table 2 in Part 2.