Geologists are scientists who study the Earth. They search through rivers, mountains, and caves for clues about how and when the Earth was formed. Some of these clues are millions of years old. Geologists study clues like caves and fossils to find out more about the Earth.
Caves are natural holes or tunnels that go underground. A geologist would study a cave to find out several things. First, what made the tunnel? Some caves are made by water or lava. Second, what kind of rock is the cave made of? Caves can be made out of many different types of rock.
Sometimes, geologists find fossils buried under the ground. A fossil is a bone, shell, or footprint left from ancient animals or people. Fossils are good clues for geologists to study. If a geologist finds the fossil of an ancient fish in a rock, he or she can guess that a river or ocean used to flow nearby.
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In a short summary of this passage, which idea would be important to include?