These clues help them establish the relative ages of sedimentary rocks. They give you a general understanding of how layers of sedimentary rocks were initially deposited. In other words, they can help you determine when and where each layer formed. Once you know these general principles, you can apply what you know to the rocks and formations around you and impress your friends with your apparently vast knowledge of geology. We'll start with the principle of original horizontality .
Geologists know that sediments are deposited in horizontal beds.
As the sediments settle and accumulate on the sea floor, they are lying horizontally. The area that's now the Grand Canyon was once under lots of water, and the sediments that settled out of the water formed the layers that we can now see on the canyon walls. A vertical sedimentary rock is a clue that something happened that was powerful enough to move the rock from its original position. For example, say you find a sedimentary rock that is turned up on its side. Well, you have to dig around for clues to figure out what moved the rock.