Sedimentary rocks form in layers. These layers were once environments as the Earth's surface. For instance, a beach may form a layer of sandstone if the sand grains become cemented together and preserved as sedimentary rock. A swampy floodplain might become a layer of shale.
Layers of sedimentary rock are found at the Earth's surface and deep underground as well. The layers that are deep underground today were at the surface when they formed long ago. The layers of rock that are on top of them are younger (unless the whole stack of rock has been overturned by tectonic processes). This is called the Law of Superposition.
People drill wells deep through layers of sedimentary rock for a variety of purposes such as to find water, oil or natural gas. When a well is drilled, sometimes the geologists use a special drilling bit to take rock out of the hole in one piece. The cylinder of rock is called a core and tells what the layers of rock are like at that one point on Earth.
In this activity, students will work with a model of sedimentary layers. They will take core samples to understand their model as they would in a real-world situation. Based on their core samples, students will deduce the relative ages of the rock layers. To provide a different perspective, students will then make cross sections to help them understand the structural relationships between the sedimentary layers.