Shale is a type of rock made from tiny particles of clay and mud that settled at the bottom of ancient oceans, lakes, or rivers millions of years ago. Layer upon layer of this sediment piled up, and over immense spans of time, the weight and pressure compressed it into thin, flat sheets of rock. If you've ever seen rock that splits easily into flat pieces, almost like pages in a book, you're probably looking at shale.

Shale is important for several reasons. First, it often contains fossils because the mud that formed it sometimes buried and preserved ancient creatures. Second, certain types of shale contain oil and natural gas trapped between their layers. In recent decades, engineers developed new techniques to extract this oil and gas from shale formations deep underground, which changed energy production in countries like the United States.

You can recognize shale by how it breaks: it splits along flat planes rather than crumbling or breaking in random directions. This quality makes it soft enough to scratch with your fingernail, but sturdy enough to build up mountainsides and cliff faces. Some shale is gray or black, but it can also be red, green, or brown depending on the minerals mixed into the original mud.