Ancient Landscapes Under the Zoo Care Center

A Building Site with a Long History

A zoo is planning a new animal care center on a dry, grassy hill. Before building begins, trained workers collect narrow cylinders of rock called cores. Each core brings up a stack of rock layers from below the ground. Students study photographs and records of the cores; they do not use drilling tools or enter the building site.

Geologist Dr. Chen records each layer from top to bottom. She checks that the layers have not been turned upside down or mixed by digging. In these undisturbed stacks, a lower layer formed before a layer above it. The thickness of a layer can differ from place to place. Thickness by itself does not tell students how many years a layer took to form.

Fossils are preserved remains or traces of ancient living things. Dr. Chen identifies the fossils using a field guide. The corals and sea clams in these records lived in salt water. The freshwater snails and reeds lived in shallow fresh water along a marsh. The grass roots and land-mammal tracks record grassy ground exposed to air. Her examination shows that these remains and traces were preserved where the organisms lived, rather than washed in from another environment.

Workers collect Core A near one corner of the planned building and Core B near another corner. The tables list their observations. Core A reaches four layers. Core B stops while still inside its third layer, so the rock below that point has not been examined. Students can compare the records and make predictions, but a prediction about unexamined rock still needs to be checked. Neither record includes measurements that give the layers' ages in years.