Most fossils are found in sedimentary rock, which is the most common type of rock on earth. Sedimentary rocks form when particles of sediment are deposited by water or wind, and then (in most cases) hardened or consolidated through time. Shale, sandstone, and limestone are types of sedimentary rocks that often contain fossils. Shale is a fine-grained rock formed from clay, and often separates into flakes or sheets. It is often grey, although it can be black, brown or red. Sandstone is a coarser- grained rock made of cemented sand grains, varying in color from cream to grey to rusty brown. Siltstone is composed of sediment particles intermediate in size between shale and sand-stone. Limestone is formed from calcium carbonate, which may precipitate chemically from water or derive from shell fragments of many tiny marine creatures. It is often beige or tan. An especially pure and soft form of limestone is known as chalk. Dolomite is similar to limestone, but with the calcium partially replaced by magnesium. For more information on sedimentary rock types see: http://marcellus.com/guide-shale-limestone-sedimentary-rocks/
Occasionally fossils occur in materials other than sedimentary rock, such as ancient tree resin (amber), in which insects or other small animals became stuck and entombed. Similarly, bones of mammoths, saber-tooth cats, and other extinct animals have been found in tar pits, in which unfortunate animals became trapped and mired. Fossils in amber or tar are often preserved with extreme detail. In arid environments, fossils may be found in a dried, mummified state in loose soils or sands. Likewise, in cold regions organisms may be frozen and preserved largely intact in frozen ground or permafrost for millions of years. Fossils also may be found in round or oval nodules called concretions, which may be split carefully with a hammer, or by repeated freezing and heating. Types of rock that normally lack fossils are metamorphic rocks (formed from sedimentary rock under heat and pressure), and igneous rocks (the result of molten or volcanic rock that later cooled). The scarcity of fossils in these rocks are due to the harsh manner in which they form, which usually destroys or severely deforms any remains of living things.
Fossils are more common than many people think. Even though a relatively small proportion of plants and animals that live are preserved as fossils, because so many animals live and die, and because sedimentary rock is very common, finding fossiliferous (fossil-bearing) rocks is not very difficult. Although some sedimentary rocks are barren (largely devoid of fossils), most contain at least some fossils, and some are quite rich in a variety of fossils. In some localities rocks are composed almost entirely of fossils or fossil fragments. Sites with exceptionally abundant or extremely well- preserved fossils are sometimes referred to as Fossil-Lagerstatten.
Many collecting sites are described in books and field guides. Some of the more popular books are listed in the bibliography. Although some sites are visited by many collectors each year, many new fossils may be exposed each spring, due to the effects of erosion, rocks falling from higher areas, new excavations (if the site is a quarry), and (in northern regions), the effects of repeated thawing and freezing each fall and winter (which tends to split rocks).
The mention of a site in a guide book does not necessarily mean that permission is given to collect there. Again, always check with the owner, and be sure to discuss any ground rules for collecting (times and places to collect, any fees due, permits or waivers to sign). Strictly abide by the owner's policies. Do not dig, trespass, or park cars except where allowed. Before leaving a site, please pick up any litter, close any gates that were opened, and fill in any unsightly or potentially dangerous holes made during your digging. In general, be thoughtful and courteous. Otherwise, you and others may not be allowed back. Many sites have been closed because of the behavior of a few thoughtless collectors.
Collecting sites also may be found through local fossil clubs, geologic societies, and museums, many of which hold regular meetings and field trips. A list of some of the active organizations in the eastern U.S. is provided in the appendix. Many high schools, colleges, and universities also sponsor trips to fossil sites in conjunction with earth science classes. Collecting with a group provides greater safety and comradery, and increases the chances that someone will be able to identify any unusual specimens found. Fossil collecting is forbidden in most state and national parks, but some allow limited collecting (check with the park office). Collecting on federally owned public lands is governed by the Bureau of Land Management (Department of the Interior) and any state and local regulations. Until recently most states permitted at least limited collecting on public lands outside of parks, provided that no machinery is used, that the collected material is not for commercial use, and that the quantities collected are small. However, many state and local regulations are currently being reviewed or rewritten, and several federal bills are pending in the U.S. Congress that could affect (and possibly severely restrict) collecting on public lands. By the time you are reading this, one or more federal bills may have passed. Always check in advance with pertinent authorities for the latest collecting regulations at each site.