Sesson 1: Environments of Deposition

Key Idea

Earth is dynamic, and landscapes change drastically over time. A location that is now dry land may have once been underwater. Geologists study rocks to uncover this hidden history.

By observing the specific properties of a rock, scientists can make an inference—a logical conclusion based on evidence—about the past. The rock serves as a record of the environment that existed at the specific time and place where the rock formed.

Key Idea

The physical characteristics of sediment (such as size and shape) depend on the energy of the environment that moved them. High-energy environments, like fast-flowing rivers, can carry and deposit large, heavy sediments (like pebbles). Low-energy environments, like deep oceans or calm lakes, can only carry small, fine sediments (like clay or sand).

Therefore, the size of the sediment grains gives a direct clue about the energy level of the past environment.

Key Idea

Because different environments deposit different types of sediment, they create different types of sedimentary rock. A rock made of large, rounded pebbles indicates a past environment with fast-moving water. A rock made of fine, smooth clay indicates a past environment with still water.

By identifying the type of rock, geologists can reconstruct the past environment, determining whether a place was once a river, a beach, or a deep ocean floor.