Some very straightforward principles are used to determine the age of fossils.
Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age.
Let's answer the question "what is a fossil" in a little more detail by going into a specific example about fossils Think about someone finding a Native American arrowhead in a recently ploughed farm field.
A fossil is the preserved remains or trace of a plant or animal from the past. " Plants can be preserved with a carbon film on a piece of shale, an animal bone can be naturally replaced by minerals and preserved, or a footprint in the sand can harden into a fossil. Sediments have to accumulate over the organism in order to preserve it.The development of radiometric dating techniques in the early 20th century allowed geologists to determine the numerical or "absolute" age of the various strata and thereby the included fossils.Like extant organisms, fossils vary in size from microscopic, even single bacterial cells one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons.Once they are able to manipulate the cards into the correct sequence, they are asked to do a similar sequencing activity using fossil pictures printed on "rock layer" cards.Sequencing the rock layers will show students how paleontologists use fossils to give relative dates to rock strata. This is an informational tour in which students gain a basic understanding of geologic time, the evidence for events in Earth’s history, relative and absolute dating techniques, and the significance of the Geologic Time Scale.