Radiometric dating and the geological time scale

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When we find evidence of a hiatus in the stratigraphic record we call it an unconformity.An unconformity is a surface of erosion or non-deposition. Because of the Laws of Stratigraphy, if we see a cross section like this in a road cut or canyon wall where we can recognize an angular unconformity, then we know the geologic history or sequence of events that must have occurred in the area to produce the angular unconformity.Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits. Stratigraphy = the study of strata (layers) in the Earth's crust.

This is the same principle used to determine relative age in the trash pits discussed previously.

Angular unconformities are easy to recognize in the field because of the angular relationship of layers that were originally deposited horizontally Disconformities (called parallel unconformities in your lab book) are much harder to recognize in the field, because often there is no angular relationship between sets of layers.

Disconformities are usually recognized by correlating from one area to another and finding that some strata is missing in one of the areas.

Notice how if we are only examining a small area in the figure above, we would determine a different type of unconformity at each location, yet the unconformity itself was caused by the same erosional event.

one based on physical characteristics or material properties of the rocks - Rock Stratigraphic Units, and the other based on the time over which the material was formed - Time Stratigraphic Units.

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