P259-65 Young.Vp

Article How Old Is It? How Do We Know? A Review of Dating Methods— Part One: Relative Dating, Absolute Dating, and Non-radiometric Dating How Old Is It? How Do We Know? A Review of Dating Methods— Part One: Relative Dating, Absolute Dating, and Non-radiometric Dating Methods Davis A. Young Davis A. Young The essential ideas behind the major methods for assessing the relative ages of geological and archeological materials and events are reviewed. These include the principles of original horizontality, superposition, inclusion, cross-cutting relations, and cross-dating by index Since [the fossils (biological succession) or artifacts. Some general principles of absolute dating are introduced, and, as representatives of non-radiometric methods, tree-ring, thermoluminescence, nineteenth obsidian hydration, and amino acid racemization dating are discussed with examples. century], ntil the mid-eighteenth century, is younger than another stratum? How can researchers in UChristians assumed that the history of we be confident that a certain granite intru- a wide range of the human race could be deciphered sion in the Sierra Nevada of California is from Scripture, monuments, artifacts, and 82 million years old rather than 80,000 years disciplines have documents. No one conceived that the Earth old or that an artifact is 12,000 years old had a very lengthy pre-human history. After (absolute dating)? developed all, Genesis presumably taught that God created the Earth mere days before he created In this first part of a three-part series, we a host of Adam and Eve. Martin Rudwick has docu- begin to answer these questions in a very mented that, throughout the late eighteenth brief review of fundamental principles of methods for and early nineteenth centuries, “savants” and relative and absolute dating and of non- 2 determining “natural historians” began to realize that radiometric methods. The second article the Earth does have an extensive pre-human will deal with radiometric dating methods both “relative” history that can be deciphered from such used for determining ages of crystallization natural “documents” and “monuments” as of minerals, rocks, and meteorites. The final and “absolute” rock strata and fossils.1 In the nineteenth article will address radiometric dating that century, archeology and paleo-anthropology involves cosmogenic nuclides, fission-track ages of rocks, began to extend the timescale of human dating, and U-He dating, and will also con- minerals, [etc.]. history. Since then, researchers in a wide sider some theological implications. range of disciplines have developed a host of methods for determining both “relative” After graduating from Princeton University, Davis A. Young received a M.S. and “absolute” ages of rocks, minerals, degree from Penn State and a Ph.D. degree in geological sciences from Brown fossils, trees, pottery, tools, occupation sites, University in 1969. Dave then taught geology for five years at New York art objects, and much else. University, five years at the University of North Carolina at Wilmington, and 26 years at Calvin College. Upon retirement from Calvin in 2004, Dave and What, then, are the grounds on which we his wife Dottie moved to Tucson, Arizona, where they enjoy the view of the can assert that a particular body of rock is Santa Catalina Mountains and Dave vigorously pursues his hobby of birding. older than some other body of rock (relative A fellow of ASA, Dave continues service as a member of the Editorial Board of dating)? And how do we know that a partic- Perspectives on Science and Christian Faith. He and his wife are members ular stratum in an ancient Near Eastern tell of Catalina Foothills Presbyterian Church in America. Volume 58, Number 4, December 2006 259 Article How Old Is It? How Do We Know? A Review of Dating Methods— Part One: Relative Dating, Absolute Dating, and Non-radiometric Dating Relative Dating in applying superposition given that stacks Relative dating was first applied to rock of sediment layers at archeological sites are strata in the mid-seventeenth century by a commonly subject to disturbance such as devout Christian anatomist, Niels Stensen excavation of burial sites into older sedi- (Steno) in his formulation of several basic ments or reworking by organisms. stratigraphic principles that are still rou- Steno also introduced what we might call tinely applied by all geologists and, in a principle of inclusion. A pebble, a crystal, modified form, by archeologists.3 or a fossil normally had an existence that preceded its incorporation into a developing Relative dating The principle of original horizontality takes mass of rock. There can be exceptions, its clue from the observation that modern however. Some crystals, for example, grow principles of sediment layers, with exceptions such as the within a pre-existing rock at the expense of dipping sand layers in dunes and sediments surrounding material. In such cases, crystal that accumulate on moderate slopes, are great importance growth may cause visible deformation of commonly deposited very close to the hori- the surroundings. Competent geologists can zontal. In applying this principle to layered to geology are: normally determine whether an included sedimentary rocks such as sandstone, con- object pre-dates or post-dates its host rock. glomerate, and limestone, one may safely Similarly, a stone tool, pot, or figurine is •the principle of infer that a stack of such rock layers that is generally older than the sediment layer that tilted (dips) at a fairly steep angle must have was deposited on and around it unless it can original experienced an episode of tilting subsequent be demonstrated that it was subsequently to its original deposition as approximately buried in a lower layer. horizontality, horizontal layers. Because layered deposits of human origin, such as trench fillings, The principle of cross-cutting relationships is •the principle of may be oriented at a steep angle, original especially important in deciphering tempo- horizontality does not always hold for layers ral relations of igneous, metamorphic, and at archeological sites. vein rocks. When the principle was first for- superposition, mulated is unclear, although James Hutton The principle of superposition states that, certainly used it in his claim that granite is in a stack of undisturbed layered rocks that •the principle of an intrusive rock. In 1785, Hutton observed were deposited on the surface of the Earth veins and dikes of granite projecting finger- (sedimentary rocks or lava flows), any given inclusion, like across the oriented structures of the sur- layer must have been deposited before rounding gneisses and schists in Scotland’s the layers above it, and is, therefore, older. Glen Tilt. From these relations, he inferred •the principle of Similarly, the layer must have been depos- that the granite veins must be younger than ited later, hence is younger, than layers the gneisses. The principle maintains that a below.4 For example, there is no field evi- cross-cutting feature which cuts across layering or any dence that the layered rocks of the Colorado other oriented structure of the surrounding Plateau, of which the Grand Canyon region relationships, rocks must have been emplaced after the is a small part, were flipped upside down. surrounding rocks were already in place Therefore, the Tapeats Sandstone at the and (Figure 1). This principle also applies to bottom of the horizontally layered sequence archeological sites. For example, the founda- exposed in the canyon walls must be older tions of walls may have been set by excava- •the principle of than all the layers above from the Bright tion into a lower substrate and have the Angel Formation to the Kaibab Limestone at appearance of transecting the stratification the top of the canyon walls. The Redwall biological immediately beneath. Limestone, a unit forming a prominent cliff (or fossil) halfway up the canyon walls, must be older Although a skilled geologist can quickly than the strata of the overlying Supai Group assess the probable sequence of events that succession. and younger than the underlying Muav affected the rocks exposed in an outcrop or Limestone. The principle of superposition is road cut from all four of these principles, also routinely applicable to any stratified there is an additional principle of great im- archeological site such as the ancient Near portance to geology, the principle of biological Eastern tells of Iraq or Israel, Native Ameri- (or fossil) succession. Similarly, archeologists can mounds, deposits flooring a rock shelter, may talk of cross-dating with index fossils and deposits on the floor of a cave. or artifacts. During the second half of the Archeologists must exercise extreme caution eighteenth century, several students of the 260 Perspectives on Science and Christian Faith Davis A. Young Earth such as Georg Füchsel and Giovanni Arduino recog- ing distinctive fossil remains. Thus, assignment of Carbon- nized that rock strata typically contain characteristic suites iferous age to a group of strata on the basis of its fossils of fossil remains by which they may be distinguished from means that the rocks in question are younger than under- other strata. Between 1790 and 1810, William Smith in lying Devonian rocks and much younger than underlying England, and Georges Cuvier and Alexandre Brongniart Cambrian strata, and much older than overlying Tertiary in France, developed this principle. In conjunction with rocks. The principle of biological succession has its archeo- the principle of superposition, Cuvier and Brongniart logical counterpart. Widely separated sites might be tem- especially showed that distinctive suites of fossils can be porally correlated thanks to the occurrence of fossil bones used to determine the relative ages of rock layers within from identical species at each site. Correlation can also be a given region. Before long, geologists attempted to corre- done with cultural artifacts such as distinctive styles of late rock layers in different areas.

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