Three Road Cuts, One Ancient Sea

Fossil Hunting Along Ridge Road

1On a cool October morning, Ms. Rosalind Achebe's eighth-grade class rode a bus along Ridge Road with Dr. Tomas Varga, a geologist from the state survey. Long ago, workers blasted through the hills to build the road, leaving tall walls of bare rock called road cuts. The class had a permit to collect fossils at three stops. Everyone wore a hard hat, stayed at least 3 m from the rock walls and collected only from the piles of loose rock at the bottom.

2At each stop, Dr. Varga pointed out the layers and helped students match the loose pieces to the layer each had fallen from. The layers at all three stops are flat, with no signs of folding, tilting or faults. Most of the rocks are limestone, shale and sandstone that formed on the floor of a shallow sea. Students filled bags with shells of brachiopods, the stems of sea animals called crinoids, corals and pieces of trilobites.

3Back on the bus, Keziah Lindqvist asked how anyone could tell the age of a rock. Dr. Varga handed out a range chart from a field guide. Geologists build charts like this by finding the same fossil in many places, including in layers that also contain volcanic ash that can be dated. Each fossil species lived for only part of Earth's history. The chart shows the oldest and youngest rocks in which each species has been found. The abbreviation mya means million years ago.

4"When two species appear in the same layer," Dr. Varga explained, "that layer must have formed while both of them were alive. Their ranges overlap only for part of the time, so you can narrow the age." He also told the class that Earth is about 4.6 billion years old, which is 4,600 million years.

5Keziah noticed that one fossil, brachiopod M, was in her bag but not in the guide's list for other states. Dr. Varga said it has been found only in this one valley.