Figure 2.1. Rocks and time. An outcrop of rock is shown, and the ages of several of the layers are given. Note that the amount of time represented is not equivalent to the thickness of the rock understanding of [geological] time comes easily enough - I know how many zeros to place after the 10 when I mean billions. Getting it into the gut is quite another matter."2 The popular writer, John McPhee, used the expression "deep time," a phrase that is redolent with the antiquity, mystery, richness, and unfathomable extent of earth history.

Geologists generally signify time in two ways: in numbers of years before present, and by reference to blocks of time with special names. For example, we say that earth was formed 4.6 billion years before present, meaning that it was formed 4.6 billion years ago and is thus 4.6 billion years old. Unfortunately, learning the age in years of a particular rock or fossil is not always easy, or even possible. For this reason, time has been broken up into a hierarchy of time intervals, and rocks and fossils can be referred to blocks of time, depending upon how well the age of the rock or fossil can be estimated.

Age of rocks Geologists are happiest when they can learn the age of a rock or fossil in years before present, a determination that is called its absolute age.

7,000 yr 8,500 yr

9,000 yr

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