Fossil collecting

Process

Locating fossils

Rock type

Fossils are generally found in sedimentary rock with differentiated strata representing a succession of deposited material. The occurrence of fossil bearing material depends on environmental factors before and after the time of preservation. After death, the first preserving factor is a rapid burial in water bodies or terrestrial sediment which would help preserve the specimen. These rocks are usually termed clastic rocks and are further divided into fine, medium and coarse grained material. Fossils can be found in all grain types, but more detailed specimens are found in the fine grained material. A second type of burial is the non-clastic rock, where the rock is made up of the precipitation of compacted fossil material. Types of non-clastic rock include limestone and coal. The third fossil bearing material is the evaporates, which precipitate out of concentrated dissolved salts to form nodular deposits. Examples include rock salt and phosphate concentrations. The evaporates are usually associated with gastropod, algae, vertebrate, and trace fossils. Fossils are not found in areas of igneous rock (except in some beds between lava flows). In rocks which have undergone metamorphism, fossils are generally so distorted that they are difficult to recognize or have been destroyed completely.

Preservation

After burial, various factors are at work that can endanger the current fossil's preserved state. Chemical alteration can change the mineral composition of the fossil, but not its appearance. Lithification can distort its appearance; the fossil may be fully or partially dissolved leaving only a fossil mold.

Exposure

Areas where sedimentary rocks are being eroded include exposed mountainous areas, river banks and beds, wave washed sea cliffs, and engineering features like quarries and road cuts. Coal mining operations often yield excellent fossil plants. The best ones are found not in the coal itself but in the associated sedimentary rock deposits called coal measures.

Wave-washed sea cliffs and foreshore exposures are often good places to search for fossils, but always be aware of the state of the tides in the area. Never take chances by climbing high cliffs of crumbling rock or clay (many have died attempting it). Dried up natural lake beds and caves in the form of pitfall traps frequently have high concentrations of fossils (e.g., Cuddie Springs and Naracoorte Caves in Australia).

Most fossils have either a different colour from the surrounding rock (due to the differing mineral composition), a differing shape or texture, or a combination of both. A fossil can be extracted from its environment with a similar colour to the surrounding rock it was found in.

Dredge-spoil islands in the lower Cape Fear River estuary near Wilmington, North Carolina expose reworked material from the Castle Hayne Limestone formation alongside Miocene and Pliocene marine sediments, making fossilized shark teeth accessible along the shoreline at low tide.

Collecting techniques

Fossil collecting techniques vary depending on the rock in which the fossils are found. For collecting in hard rock, a geologist's hammer, a variety of cold chisels, and a mallet are used to split and break rocks to reveal fossils. Because the rock is deposited in layers, these layers may be split apart to reveal fossils. For soft sediments and unconsolidated deposits such as sands, silts, and clays, a spade, flat-bladed trowel, and stiff brushes are used. Sieves in a variety of mesh sizes are used to separate fossils from sands and gravels. Sieving is a rougher technique for collecting fossils and can destroy fragile ones. Sometimes, water is run through a sieve to help remove silt and sand. This technique is called wet sieving.

Fossils tend to be fragile and are not usually extracted entirely from the surrounding rock. Cloth, cotton, small boxes and aluminum foil are frequently used to protect fossils from damage while being transported. Occasionally, large fragile specimens may need to be protected and supported using a jacket of plaster before their removal from the rock. If a fossil is left in situ, a cast may be produced using plaster of paris or latex. Such a cast is inexpensive, easier to transport, causes less damage to the environment, and leaves the fossil in place for others to examine (though it does not preserve every detail). Fossilized tracks are frequently documented with casts. Subtle fossils which are preserved solely as impressions in sandy layers, such as the Ediacaran fossils, are also usually documented by means of a cast, which shows detail more clearly than the rock itself.

Preparation and cleaning

For smaller fossils, a stiff brush is sometimes used to dust and clean the fossil. For larger fossils, a chisel can be used to remove large bits of dirt. However, this runs the risk of damaging the fossil. Running water can cause some types of fossils to either dislodge from the rock or even crumble and break apart, for they are very fragile. Dental tools are sometimes used to remove small amounts of rock from the fossil.

Documentation

A knowledge of the precise location a fossil is essential if the fossil is to have any scientific value. Details of the parent rock strata, the location of the find, and other fossil material associated with the find help scientists to place the fossil in the context of the time, location and situation in which the organism died. Data logs, photographs, and sketches may accompany detailed field notes to assist in the locating of a fossiliferous outcrop. Individual fossils are ideally cataloged with a locality number and a unique specimen number. This allows for a collection to be easily searched and for specimens to be located. Cataloging of collections is almost universal in large institutions like museums.