Cliffs act as natural history books where each rock layer represents millions of years of Earth's past.

The study of rock layers is known as stratigraphy, a branch of geology established by Nicolas Steno in the 17th century. His Principle of Superposition states that in an undisturbed sequence of rocks, the oldest layers are at the bottom and the youngest are at the top. This allows geologists to reconstruct Earth's history by reading the layers like a chronological archive.The Grand Canyon in Arizona provides one of the most complete records of geologic time on Earth. The Vishnu Basement Rocks at the canyon's floor date back approximately 1.75 billion years, while the Kaibab Limestone at the rim is about 270 million years old. This massive gap represents nearly 40% of Earth's entire history visible in a single location.Chemical compositions within these layers provide specific clues about past environments. For instance, the presence of iron oxide (hematite) creates the vibrant red hues seen in the Redwall Limestone, indicating an oxygen-rich environment. Conversely, layers containing high amounts of calcium carbonate and marine fossils like brachiopods suggest the area was once a shallow tropical sea.Modern geologists use radiometric dating to assign absolute ages to these layers by measuring the decay of radioactive isotopes like Uranium-238. This precision allows the International Commission on Stratigraphy to maintain the Geologic Time Scale. By analyzing these strata, researchers can pinpoint the exact timing of mass extinctions and major climatic shifts throughout the Phanerozoic Eon.