Have you ever noticed that some rocks appear to have stripes or layers? Sedimentary rocks, which form when particles settle out from water or wind and later, over time become rock, often have distinct layers that geologists call strata. Imagine making a peanut butter and jelly sandwich, which is also made up of layers- the bottom slice of bread is placed on the plate first, followed by a layer of peanut butter, then jelly, then the top slice of bread. Whether looking at a sandwich or a rock formation, we know that layers on the bottom were formed first (are the oldest), and the layers on the top were placed there last (are the newest). This principle is called the Law of Superposition, and it helps us to understand the relative ages of rocks and the fossils that are found in them. Sometimes these layers remain flat, but other times they are tilted. What does it mean if they are tilted? It means that they were once flat, but some other force (such as a fault) acted upon the earth to tilt the rock. Once you know this, you can start to "read" the story of the landscape around you! Geologists study the science of strata, called stratigraphy, to better understand how and when events in Earth history happen.
Can you find layers in the rocks of your home town? If you don't see rocks, perhaps you can see layers in dirt or sand or a stream bank, perhaps you can find them in a building, or even a stack of books. The key is layers, with older stuff on the bottom and younger stuff on the top. Show us what you see! Send your pic to usgs_yes@usgs.gov or use #findafeature and tag us @USGS_YES on social media.
Sharing/Privacy
We'll be watching Instagram and Twitter for some great #findafeature examples and may share them here with the first name or initials of the contributor, and a general location. If you tag us with @USGS_YES you are giving us permission to use your image. Please see the USGS social media sharing policy at: https://www.usgs.gov/copyright-permission-agreement-social-media-submissions. Or, you can e-mail photos to us at usgs_yes@usgs.gov and we may share them on this page or on social media. Thanks for participating and for seeing science all around you!
