How did fossils of sea creatures come to be in the middle of a desert? Evidence suggests that the land in Texas was once not a mountainous desert at all. Instead, geologists, or scientists who study Earth, believe the area was once covered by a shallow, salty sea. Eventually, the sea waters receded, and dry land covered more and more of the area that is now Big Bend. Over time, erosion has helped form narrow canyons along the path of the Rio Grande river. Just as we learn about trees by studying the rings in their trunks, scientists examine layers of rock in the ground, called strata, to learn about geological formations. They use these layers to infer what the environment was like during certain periods. Sedimentary rocks, including limestone, sandstone, and shale, contain many different kinds of minerals.

Which information would support the idea that scientists must understand how rock layers are formed to understand how the environment changed?

a
Sedimentary rocks, including limestone, sandstone, and shale, contain many different kinds of minerals.
b
Evidence suggests that the land in Texas was once not a mountainous desert at all.
c
They use these layers to infer what the environment was like during certain periods.
d
Over time, erosion has helped form narrow canyons along the path of the Rio Grande river.