Frequent references will be made to the geologic times-cale in succeeding chapters and a summary of geologic time is provided on the inside front and back cover of this volume. In many ways, the naming of rock units is similar to the naming of organisms, in that the geologic timescale is not fixed, but is constantly updated and refined by the International Commission on Stratigraphy (ICS), part of the International Union of Geological Sciences (IUGS). At the formation level, there is a type section of that formation, where rocks that are typical for that formation are exposed. Periods are formally defined at the base of each period by a Global Stratotype Section and Point (GSSP); the GSSP is designated on the ICS timescale as a golden spike. The global stratotype section for the base of a period is somewhere in the world where an excellent section of rocks is exposed and is agreed upon by a committee of experts. For more information, please refer to Gradstein et al. (2004) or the ICS web page, www.stratigraphy.org.
There are two types of units in geologic time: rock units and time units. Rock (lithostratigraphic) units refer to the physical rocks themselves and the terms, Lower, Middle, and Upper are used for these, for example this fossil was found in Lower Devonian rocks. Time (chronostratigraphic) units refer to the period of time represented by those rocks, and use time units (i.e., Early, Middle, and Late), for example, plants of the Early Devonian.
Throughout this book, you will see the following abbreviations for geologic time:
ka, Ma, Ga—These stand for, respectively, thousands of years, millions of years, and billions of years (gigayears) before present. These are used for dates, for example this plant lived 450 Ma (450 million years ago).
kyr, myr, byr (sometimes written Gyr)—These stand for intervals of time, for example this group survived for 20 myr.
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