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Download Index Fossils - Hicksville Public Schools
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Index Fossils • Index fossils pinpoint the time period during which a rock layer formed. • Good index fossils meet the following requirements: –Distinct body features –Abundant –Widespread range or distribution –Existed for a short period of time Examples of good index fossils Trilobites Graptolites Ammonites KEY BEDS • Well defined, easily identifiable layers or formations that have distinct characteristics that enable them to be used for correlation – Iridium Rich Layer (iridium is commonly found in meteorites, but not on earth) – Volcanic Ash deposits Purcell Sill • Quite often key beds, like the rocks above and below them, are sedimentary in origin, but they can also be layers of ash deposited over a large geographic area by ancient volcanic eruptions So, how do we use fossils to date sedimentary layers? • Without fossils, layers from different outcrops can be correlated, but this can be difficult if there is an unconformity, or when different strata are exposed in different locations. • They help to determine if there is an unconformity . Index fossils help us to determine the time period that a layer was formed.
