Solid as they may seem, rocks are constantly being eroded and reformed in a process called the rock cycle. Nowhere is the geologic history of Earth made plainer than in the Grand Canyon, where rock layers offer us clues to life over the past 1.2 billion years. Thanks to glaciers, another geologically revealing place is Yosemite Valley,
where ancient granite walls stand exposed by the ice that carved out the valley millennia ago. Core samples and fossil finds in places such as the Grand Canyon and Yosemite tell of Earth’s geologic past
and the evolution of life. Once humans arrive on the scene, rocks take on new stature as objects of art and value. No gem was more valuable to the Maya than jade, and gold is universally prized. They are proof that the Earth holds many treasures.
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