Study GuideGeology–Geologic Time1. Geologic CorrelaƟonsGeologists often need tomatch rock layers from different locationsto figure out their age andhistory. This process is calledgeologic correlation. Because Earth’s rock record israrely completeor fully exposed, geologists rely on several clues to decide whether rocks in different places formed atthe same time.1.1 Physical ConƟnuity: Matching Rocks by LocaƟon and Layers1.1.1 Tracing Rock LayersThe simplest way to correlate rocks is throughphysical continuity. If a rock layer or boundary can beseen continuouslyfrom one place to another, it is easy to prove that the rocks are the same.However, this is rarely possible over long distances. Erosion, vegetation, and burial often break uprock exposures. As a result, geologists usually mustcompare rock sequences in detailrather thantrace them directly.1.1.2 Comparing Rock SequencesGeologists examine:•Therock type•Theorder of layers•The rocks foundabove andbelowthe layer being studiedFor example, if you find this sequence in two different places:•Red shale•Sandstone•Green shale•Limestone•ChertPreview Mode
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