Reading the Gorge Wall: Layers, Fossils and Deep Time

A Hike Through Time

1On a spring field trip, Ranger Teodoro Vasquez led an eighth grade class from Cedar Hollow down the river trail of Stone Fork State Park. Over many thousands of years, the river has cut a deep gorge, and its walls show flat layers of rock stacked like the pages of a book. Maile Kahananui and her classmates recorded each unit from the top of the wall to the river's edge. Their notes are in the table. The class stayed on the trail and only looked at the fossils, because the park protects them and climbing the wall is not safe.

2Ranger Vasquez pointed out two thin beds of volcanic ash. Ash from an eruption settles over a wide area in a short time, and scientists can measure its age in a lab. Ash bed X, at the top of Layer 3, formed about 300 million years ago. Ash bed Y, near the base of Layer 4, formed about 200 million years ago. Above ash bed Y, Layer 4 holds dinosaur footprints.

3Between ash bed X and Layer 4, the boundary is wavy and uneven, not flat like the others. A dark band of igneous rock, called a dike, cuts straight up through Layers 1, 2 and 3 and through ash bed X. The dike formed when melted rock squeezed into a crack and cooled. It stops at the wavy boundary, and Layer 4 lies across its top without a break.

4Layer 3's limestone is packed with broken shell pieces. It holds a crinoid species that the class's fossil guide calls species K. At a limestone quarry 60 km away, geologists have found species K in a layer there too.

5Back at the visitor center, the class walked the park's Time Trail, a 460 m path that shows all 4.6 billion years of Earth's history. On this trail, 1 m stands for 10 million years. The trail starts with Earth's formation and ends at the present day at the visitor center door. Signs mark events such as the first trilobites, about 520 million years ago, and the end of the dinosaurs, about 66 million years ago.