This roadcut a few miles south of Cascade along I-15 illustrates one of the basic principles used by geologists to determine the relative ages of rocks. The horizontal layers are sandstones made up of sediment laid down near the coast of a shallow sea that was present here millions of years ago. The darker rock that cuts vertically through the layers of sandstone is type of igneous rock formed as magma solidified beneath the surface. These types of igneous formations, called “dikes,” are common in the Cascade area.
Relative age. . .
Absolute age . . .
terms: inert, radioactive decay
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Earth Science Teacher at Helena High School