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What Rocks Are Fossils Commonly Found In?

Fossils are most commonly found in sedimentary rock, the kind of rock formed by particles of sediment that are deposited by water or wind and harden over time. For obvious reasons, this process of slow accumulation is much more conducive to the preservation of animal and plant remains than the hot and violent processes that shape harder types of rock like metamorphic and igneous rock. Fortunately for fossil collectors, sedimentary rock is the most common rock type on Earth and a number of varieties offer prime fossil-finding opportunities.

Shale
- Shale is a soft, fine-grained rock formed from hardened clay and other minerals. Shale often breaks into flakes when you hold it in you hand and is usually gray, although red, brown and black variations exist depending on the other minerals in the mixture. As a soft rock made mostly from surface minerals like clay, shale forms very quickly and can harden around a dead animal or plant shortly after the organism dies, thus preserving the remains and often holding fossils in excellent condition.
Sandstone
- Compared to shale, sandstone is much coarser-grained and more solid. As the name suggests, sandstone is formed by compressed grains of sand and varies in color from cream to gray and light brown. Like most sedimentary rocks, sandstone forms in layers as geological conditions change and different times allow more particles to settle on the surface. These layers can provide a valuable record of geological history and, particularly when fossils can be found and dated in the layers, can help illustrate the history of life on Earth between the natural processes that formed the planet's surface.
Limestone
- Limestone is formed mainly by deposits of calcium carbonate, a compound often found in water and in the shells of marine organisms. Pure limestone is usually a variety of shades of white, but can also be darker colors depending on the sediments founds in the stone. Due to its marine origins, most fossils found in limestone are of aquatic plants and animals. In fact, many of the ridges found in limestone are the remains of marine organisms like corals and crustaceans.
Others
- While fossils are most often found in sedimentary rock, some fossils can be found in metamorphic rocks like marble and, especially in extreme temperatures, in a variety of other settings. In very arid climates, fossil remains can be preserved in loose sands, just as they can be preserved in ice in very cold climates. Lastly, some fossils can also be found in oils and resins like tar pits and hardened tree resin (amber). In many cases, fossils preserved in deserts, ice, amber or tar are especially well preserved.

Fossil Collecting