Sediment cores are one of the most valuable types of samples for researchers who would like to learn about past climate or ecological changes. Cores can be retrieved from lakes, marshes, swamps, fields, and the ocean. The layers often reveal striking changes in color (see photos) reflecting changing sediment composition (i.e. more clay deposition or more microfossil s). This easy activity illustrates the basic geologic principle that horizontal layers of sediment become older the deeper you go below the Earth’s surface (Law of Superposition). Each layer contains sediment, fossils and organic matter etc. that can inform us about past changes in the associated water mass, and these changes are commonly associated with changes in the environment. This is also a way to demonstrate how geologists extract cores from lake bottoms or other areas.