As best that scientists can figure, the first rocks formed on the earth about 4.5 billion years ago. The earth is maybe 6 billion years old, but took it took a while for the crust to cool off and solidify. The rocks of the earth are the key in determining not only how the world is put together, but how old it is. You've seen places on the side of the road where the rocks lay in horizontal beds on top of each other. These are sedimentary rocks. In those parts of the world where the layers haven't been jumbled together by mountain-building forces, the bottom layers are older. Like a detective, scientists read the clues and can tell the ages of the rocks by the fossils in the layers. Since the 1950's, special techniques have been devised by geologists using radioactive isotopes present in rocks to accurately age date sedimentary, igneous, and metamorphic rocks. Guess what! The age dates correlate well with that of the fossil record found in sedimentary rocks, adding support to the acceptance of the Geologic Time Scale. So, by looking at road cuts, quarries and sections of rock cored by drilling, geologists pieced together a history of the planet.
I recently drove through this bypass area on Highway 63 in Hardy, AR. I was amazed at the work that had been done to create a bypass through this rock but also enjoyed viewing the make-up of the rock itself. You can see all of the different colors of sediments in this rock. You can see the limestone, silt stone, and sandstone layers throughout. Road cuts are when a mountain or a hill is cut open to make way for a road.
Northern Arkansas was a shallow south-sloping sea floor (continental shelf), the Arkansas River Valley was near the edge of the shelf, and the Ouachita area was a deep abyssal plain.
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There is a very large shoulder for cars to pull off and view the area. It is not necessary to leave the vehicle.
1) In the highway cut, you will see several layers of rock, evidence of some past extreme geological action. What is the general angle of the layers?
2) Do you see any distinct rock layers?
3) Name the kind of rock that makes up each of these layers?
4) Describe the different colors of rocks seen at your location.