This brings us to Today.
 
But how do we know this history? What are the clues? How do we know that the layers shown here in so many colors are actually
there? One important source of information is from oil geologists. The great search for oil led to the drilling of hundreds of
thousands of test wells, from which rock core samples were brought up. These cores may be as much as 10,000 feet in length.
Exploration for oil was extensive, and has produced a detailed map of what is below the surface. Now that virtually all of the earth’s
oil has been found, oil companies have donated their cores to university geology libraries for public access.
 
From these cores, from radiometric dating of different layers, from following the same strata from core to core, we can construct a
very detailed picture of the top few miles of the earth’s crust. By examining fossils from different strata, we can reconstruct the
environment in which the plants and animals lived, as well as discover the details of what the plants and animals were like.
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