How are fossils recovered from rocks?

This segment explains how fossils are found within rocks, and the various ways in which they are recovered and prepared for scientific study.

Most fossils are found in sedimentary rocks that have been formed by the consolidation of soft sediment such as sand and mud. Fossils are rarely, if ever, found in igneous rocks that have solidified from a molten state, or in metamorphic rocks that have undergone intense pressure and heat. To find fossils, it is necessary to find places where suitable rocks are exposed, such as natural outcrops in cliffs, river banks or the sides of hills, or man-made exposures in quarries, road cuttings or building excavations. The type of fossils found will depend on the age of the rocks and on the environment in which they were deposited; for example, dinosaurs are not found in rocks more than about 235 million years old or less than 65 million years old, and fossils of marine animals or plants are obviously not found in rocks which have been deposited in fresh water or on dry land.

Fossils are sometimes exposed on the surface of rocks, but better specimens can usually be found by splitting the rock along the bedding plane with hammers and cold chisels. Sometimes it is necessary to remove surrounding rock using a pick or crowbar, or even heavier mechanical equipment.

Once the fossil has been retrieved from the outcrop, it is taken back to the laboratory for further treatment. If the specimen is large and fragile, as is commonly the case for vertebrate fossils, it may be encased in a plaster of paris jacket to protect it during transport.

Once in the laboratory, excess rock obscuring the fossil can be removed. In most cases, this is done mechanically by chipping the rock away using hammers, cold chisels, needles, electric engraving tools, dental drills and rock saws or grinders. In the case of small specimens, or for exposing fine structures, this work is carried out under a microscope. Glues are used to repair breakages, or to impregnate fragile specimens in order to strengthen them. For fossils preserved in limestones, it may be possible to remove the surrounding rock by dissolving it in dilute acid, but this technique can only be used when the fossil itself is not soluble in the acid.

Some fossils are preserved in very soft rock which may break down in water, so that the fossils can be collected by washing the rock through a sieve underwater. In this way, large quantities of rock can be processed to recover fossils that would be missed by ordinary digging.

References:

Rixon, A. E. 1976. Fossil animal remains. Their preparation and conservation. University of London, London.