Dating index fossils

In the Grand Canyon, the layers of strata are nearly horizontal. Most sediment is either laid down horizontally in bodies rocks water like the oceans, or on land on the margins of streams and rivers. Each time a new layer of sediment is deposited it is laid definition horizontally methods top of an older layer. This is the principle of original horizontality:. Thus, any deformations of strata Figures 2 and 3 must have occurred after the rock was deposited.

Every fossil tells us something about the age of the rock it's found in, and index fossils are the ones that tell us the most. Index fossils also called key fossils or type fossils are those that are used to define periods of geologic time. A good index fossil is one with four characteristics: it is distinctive, widespread, abundant, and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are marine organisms. That being said, certain land organisms are useful in young rocks and specific regions. Any type of organism can be distinctive, but not so many are widespread. Many important index fossils are of organisms that start life as floating eggs and infant stages, which allowed them to populate the world using ocean currents.

Relative Dating and Index Fossils. Copy and Edit. With Super, get unlimited access to this resource and over , other Super resources.

Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy layers of rock are called strata. Relative dating does not provide actual numerical dates for the rocks. Next time you find a cliff or road cutting with lots of rock strata, try working out the age order using some simple principles:.