High School Earth Science: Evidence About Earth’s Past - Relative Ages of Rocks — Flashcards

When rock layers are located in different places, laws such as superposition and cross-cutting cannot be used to match rocks that are the same age. Matching these rocks requires other evidence.    
Sometimes distinctive rock formations may be recognizable in different locations. If so, rocks in the different locations are assumed to have formed under the same conditions at the same time. For example, the famous White Cliffs of Dover in southwest England consist of a soft rock called chalk. The rock formed from organisms that settled to the bottom of an ancient sea. Similar white chalk cliffs are also found in Denmark and Germany. They formed at the same time as the cliffs in Dover.    
Index fossils can often to be used to match rocks in different places. Rocks with the same index fossil(s) must be about the same age. To be useful as an index fossil, a fossil must represent an organism that was widespread but existed for only a short period of time. The well-known trilobite is often used as an index fossil.    
Key beds can be used like index fossils to match rock layers in different locations. A key bed is a distinctive layer of rock that is found in many areas of the world or even worldwide. A famous key bed is a thin clay layer that formed all over the world at the boundary between the Cretaceous Period and the Tertiary Period. This is when the dinosaurs and many other organisms went extinct. The clay layer contains a high concentration of iridium, an  element that is rare on Earth but common in asteroids. The iridium makes this key bed easy to identify wherever it is found on the planet. Many scientists think that a huge asteroid struck Earth late in the Cretaceous Period, blanketing the planet with iridium-rich dust. The dust may have been a major reason for the mass extinction because it would have blocked sunlight from reaching Earth. When the dust settled, it formed a thin layer of iridium-rich sedimentary rock.