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The next time you find yourself nodding behind the wheel from the monotony of turnpike driving, pull off beside a road cut where highway engineers have blasted the rock outcrop. Your reward will not only be a well-earned rest, but also the possibility of finding some of those exquisite treasures in the dust that we call fossils. Learning how to find fossils is both and entertaining and educational skill.
Other excellent hunting grounds for fossils include old rock quarries, coal and metal mines, canal and river banks, mountain slopes, and beaches. In short, wherever the bedrock of the Earth's crust has been exposed, you have a good chance of finding fossils. If you're not successful the first time you go fossil browsing by a road cut, don't be discouraged.
Francis Tuller, a beginner collector who went on to be a seasoned veteran with an enviable collection of more than 3,000 animal fossils, began by breaking open rust-brown ironstone nodules heaped beside his favorite fishing spot. One day he turned up a real prize—the fossil of a strange and grotesque 13-centimeter-long creature unknown in the paleontological records. He brought it to the attention of experts, and was eventually rewarded by having the long-extinct animal named after him—Tullimonstrum gregarium ("common Tully monster").
How Fossils are Formed
Fossils are the remains, or imprints, of animals or plants that have been preserved in rocks. They predate the withdrawal of the last glacier about 10,000 years ago. Thus, a fossil can be as young as 10,000 years. The oldest known fossils—microscopic bacteria-like organisms found in Swaziland, Africa—date back to 3.8 billion years ago.
Don't expect to find fossils in exposed rock. Virtually all fossils are found in sedimentary rock. Sedimentary rock is composed of mud, lime, or sand deposited on the floors of ancient, shallow seas. The rock may also be the dried and solidified remains of mud left behind by long-vanished rivers, lakes, or swamps. Over the vast stretches of geologic time, untold billions of animals and plants lived, died, and were fortuitously buried in soggy heaps of sediment. Slowly, the sediments were compacted, the water gradually squeezed out of them, and the individual particles eventually cemented together by minerals that acted as glue. The slow, inexorable process that converts soft sediments into solid dry rock is called lithification.
There are at least three ways animals or plants have become fossilized. First, if an organism, or part of it, was tough enough—such as bone, shell, or hard wood—it may have been preserved more or less intact if conditions were dry and the area was protected from erosion. But hardness alone is not enough. The hard part must have been buried quickly after the organism died (or while it was still alive), or it would have decayed like the soft parts of the organism. Also, the plant or animal must have remained undisturbed during the entire time it was being fossilized.
In a few unusual and rare cases, some very special condition has helped preserve almost the entire animal—either extreme cold or extreme dryness, for example. Almost entire fossil mammoths have been found preserved in frozen ground, refrigerated for more than 25,000 years in Siberia and Alaska. In dry regions of South America, parts of mummified ground sloths have been found preserved in caves. However, these cases are not the norm.
A second way in which many plants and animals have been fossilized is petrification—their bone, shell, or other hard parts were changed into a different substance. Mineral-bearing water slowly seeping downward through the sediments was soaked up by the porous bones, shells, or wood. Gradually, as the water evaporated when the sediments dried, the minerals left behind filled the small open spaces within the bone or shell. The addition of minerals tends to make bone, shell, or wood even harder. Often the actual bone or shell was dissolved by the groundwater. When that happened, the minerals in the water slowly replaced the bone, shell, or wood as they were dissolved. Brightly colored silica, calcite, or orange and red iron compounds often become part of fossil bone or shell. In some petrified wood, silica has not only filled in small hollow spaces, but has also replaced the once-living woody tissue so perfectly that the individual cells and annual-growth rings show up clearly many millions of years later.
A third group of fossils are merely traces—leaf or foot impressions in stone—of once living organisms. After the plant or animal died, it was quickly buried in the sediments where its entirety dissolved over time. Therefore, only a cavity was left in the sedimentary rock where the shell or other hard part once lay. The walls of such cavities then became a natural copy, or mold, of the shell or other skeletal part. Millions of years after the cavity was formed, minerals refilled the cavity. In this way, a natural cast of the original mold was formed. Millennia later, many such casts can be dug up by lucky fossil hunters. Molds and casts are very common fossil forms, particularly for invertebrates (animals lacking backbones).
Occasionally, the hard outer skeleton and tiny appendages of insects have been discovered in amber, which is the fossilized resin of ancient trees. Plants and small soft-bodied organisms living in the seas have been buried in mud that hardened into shale. In these cases, the only remains are a thin film of carbon showing the delicate details of their appearance.
How to Find and Prepare Fossils
You do not need much in the way of special equipment to prepare yourself for a fossil hunt. Because most fossils are found in sedimentary rocks, you'll find it helpful to learn to identify the common fossil-bearing sedimentary rocks: limestone, dolomite limestone, coal, and shale. A rock-identification guidebook is an invaluable tool, as is a geological map of the area in which you plan to search. Other tools you should take are a geologist's hammer or a bricklayer's hammer. You should also prepare one or two medium-sized tempered-cold-steel chisels, a knapsack, some old newspapers, masking tape, a small notebook, a ball-point pen, a small magnifying glass, and a few plastic pill bottles with cotton to hold small or delicate specimens.
When you are searching for specimens, avoid running quickly from one spot to another. Instead, spend some time looking carefully either by crawling slowly on your hands and knees, or by just sitting in one place. Turn over loose pieces of rock and carefully examine all sides of them. You will be surprised at what you may find. With your hammer and chisel, carefully split the sedimentary rocks parallel to the layers—not across them. It is on the broad, flat surfaces between layers that you will find fossils.
When you find a specimen, wrap it in newspaper and tape the ends together. On the tape, number each fossil and the date on which you found it. In the notebook, record the number and the date for each fossil, a short description of it, the kind of rock you found it in, and where you found it. This will allow you to return to the spot with minimum frustration in the future.
After you have brought your specimens home, clean them by placing each one in water with a mild detergent and letting it soak over night. Excess rock and soil can then be removed with a stiff toothbrush. You may then use long needles, tweezers, or old dental picks to clean around the smaller structures of your fossil. You may even remove much of the rock around the fossil with the dental pick, but do so cautiously. If a specimen is found in stained condition, soak it in Clorox overnight.
If you are a serious collector, after cleaning and drying each fossil you should label and catalog it. One way to arrange specimens is to group all the fossils from one collecting area. You should denote each collecting area with a different letter, and then paint the letter and specimen number on the fossil. Much of the information you find about the fossil should go on a special label together with the catalog number. The label should be placed beneath the fossil in its storage tray. This information along with additional data from your field notebook should be transcribed on a file card. These catalog cards, one for each fossil, can then be arranged in numerical order with each group. Keep everything in your files! A fossil without such information is little more than a curio—and has no scientific value.
A word of warning: If you plan to do any fossil hunting on private or public property, be sure to get permission. Although most public property is "open," it may be temporarily closed to rock and fossil collecting because it is under a mining claim and so on. And don't risk getting a ticket by stopping to explore a road cut when turnpike signs warn you that you may stop only for an emergency.