Fossils Are Most Common in Which Rock Types?

By Esperance Barretto
Fossils, sedimentary rock types
fossile image by danimages from Fotolia.com

Fossils are the remains of living organisms preserved in varying states within the Earth's geologic deposits. Remnants of plant and animal, covered between layers of sedimentary material like rock fragments, sand and clay, harden over time to form rocks. Fossils are found in sedimentary rocks, which are created by mechanical or chemical precipitation, and not in igneous and metamorphic rocks. Fossils cannot survive the elevated temperatures and pressures through which igneous and metamorphic rocks are formed. Limestone, sandstone, shale and conglomerate are some of the sedimentary rock types that contain fossils.

Limestone

Limestone is generally found in warm, shallow marine waters like reefs. It is composed of calcium carbonate that is biologically produced from shell fragments and remains of marine organisms like clams, brachiopods, bryozoa, algae, crinoids and corals. When these organisms living at the bottom of the sea die, their bodies accumulate in piles of debris that fossilize into limestone beds. Limestone can also be formed through a chemical precipitation of calcium carbonate from lake or ocean water.

Sandstone

Sandstone is formed in diverse environments where huge quantities of sand accumulate as a result of water or wind movements. It is generally found in rivers, beaches, flood plains, deltas, oceans and deserts. This rough-grained rock is composed of sand grains that are cemented by deposits of silica, calcium carbonate or iron oxide. Sandstone can be layered in different colors, as a result of the fossil deposits. For instance, the dark brown-colored Tapeats sandstone that layers the Grand Canyon rocks contains extinct marine species like trilobites and brachiopods.

Shale

Shale is a finely textured sedimentary rock that is formed by the compaction of silt and clay beds. It has a smooth appearance and can be separated into distinct flakes or layers. The color of this rock is determined by its organic content. For instance, the dark, rust-colored hermit shale in the Grand Canyon is comprised of plant fossils like ferns and conifers, as well as fossilized tracks of reptiles and amphibians.

Conglomerate

Conglomerates are produced in surroundings proximal to sediment and high energy transportation sources, which can move large grains. Composed of rounded, weathered pebbles, conglomerates are cemented together by minerals such as calcite, silica, clay and gypsum. Acritarch microfossils, articulate and inarticulate brachiopods, sponges and gastropods are among the numerous marine fossils occurring in the conglomerates of Canada's Western Newfoundland and southeastern Labrador Peninsula.