Fossils are most commonly found in sedimentary rocks, but all sedimentary rocks are not necessarily fossiliferous. It took me a long time to figure out why some scrumptious-looking outcrops of limestones, shales or sandstones that should have had fossils, are barren should have fossils. Nada! Not a speck or hint that anything alive had ever existed in the environments those sediments represented, when every one of my instincts told me there should the rocks were a perfect spot to find great fossils. Here are the three main reasons: Few if any creatures or plants larger than microscopic forms lived in those environments because it was toxic or otherwise unsuitable for things to live in (Ever hear of the Dead Sea?); there was an incredible amount of biological activity such as the that of soft-bodied, bottom dwelling organisms that continually turned over the sediments and prevented things from getting trapped and preserved in the forming sediments; subtle heating of the strata by nearby volcanic activity, or altering of the rocks by chemicals, one or both destroying the fossil evidence, but not enough metamorphosis to alter the appearance of the deposits. Look what happens to the chocolate chips in cookie dough when they are baked! The trick here is to keep looking because, from experience, there is some layer, maybe a thin one, or maybe at the very top or very bottom of the strata, that will hold a treasure trove of well-preserved fossils. Trust me on this point.
The following are descriptions of rocks which commonly contain fossils.
All of these rocks are found in Arizona.
Sandstones, Siltstones. These rocks can be the preserved sediments laid down in parts of slow rivers, deltas, off-shore deposits, beaches, or sand dunes. Here in Arizona, north of the Mogollon Rim, sandstones commonly preserve water ripples (right), dinosaur, reptile or amphibian tracks. In other places in Arizona, petrified wood in sandstone is common, as are dinosaur bones and hard-shelled invertebrates. The grains of minerals that make up these two kinds of rocks are usually well bonded by mineral cements like quartz or calcite, and while some can easily be split along thin bedding planes, other fossils are extracted only with a great deal of effort.
Conglomerates. Plants and animals had a difficult time surviving in the environments that produced these rocks because the large pebbles represent some pretty rugged, high-energy deposition. Many fossils were simply pulverized into dust. However, in portions of slow moving rivers, or where sediments that were carried in currents came to rest, intact fossils can be found cemented among the stones. In these rocks, especially in Mesozoic channel deposits of north central Arizona, both isolated bones and teeth, and concentrations of reptiles and amphibian fossils are frequently encountered.
Shale. Fine-grained rock such as shale often contains fossils which are either perfectly preserved or crushed parallel to the bedding plane. Shales can contain vertebrates, invertebrates and plants, or they may contain nothing if environmental conditions during deposition were incapable of supporting life. The Chinle Formation seen at right is partly composed of shale derived from river sediments and volcanic ash. The red, orange and purple hues come from iron and manganese within the shale
Limestones. Limestones are often the most fossiliferous of rocks representing the seafloor of both shallow and deep tropical seas, lagoons and some fresh-water lakes. Limestones differ greatly in their appearance and density, and fossils found within such rocks vary greatly in the ease with which they can be collected. Massive limestones often require heavy equipment, such as five or ten pound sledges and crowbars, while silty limestones give up their fossils easily, usually with nothing more needed than a pocket knife. In Arizona, from North to South, Paleozoic limestones can contain a wide variety of invertebrate fossils, as well as armored fish remains and shark teeth.
Evaporites and Marls. Fossils are very rare and usually totally absent from evaporites such as gypsum or salt, but in gypsum-rich deposits, such as the Pliocene San Pedro Formation near Benson and St. David (S.E. of Tucson), a once immense lake or series of lakes slowly drained or evaporated as climates changed, leaving massive gypsum beds. In associated siltstones, laced with amber-colored selenite crystals, well-preserved bones of Cenozoic mammals and reptiles are frequently encountered.
Coal and Coal Shales. Fossils in carbon rich shales directly associated with coal can often be exquisitely preserved. Coal is produced by distillation, a process in which volatiles in rotting accumulations of plants are lost over time, leaving relatively pure carbon. Usually plant fossils are represented, but vertebrates such as fish, insects and even dinosaur footprints have been found near coal deposits. The massive coal and shale deposits of Black Mesa (right), in Northeast Arizona are incredibly rich with Cretaceous marine invertebrates, shark teeth, and occasional marine reptiles.
Knowing where to look for specimens is probably the greatest problem a fossil hunter will encounter. The LOCALITIES page on this website covers a lot of fossil collecting localities offers and offers a relatively painless way to find collecting areas with reasonable chances of success. The localities listed are so extensive that they are not likely to be picked clean by the dozens of people who might visit them. The Southwest is blessed with extensive exposures of sedimentary rock and in many cases sediments rapidly eroded by seasonal rain and flooding; this also serves to dilute the effects of over-collecting in popular areas. Often roadcuts similar to the one on the right can be easily accessed, others are much more remote and a visit to them requires careful planning and careful attention to safety and age or health related issues. (See CHECKLIST in this section)
Many museums curate and exhibit fossils and archaeological from Arizona, and although they are often displayed for the public, usually the labels are too ambiguous to pinpoint their exact locality. But, museums are a good place to start your detective work; most have libraries and archives which are available for use by the general public and within these are often the museum's own publications and professional journals which often pinpoint known areas where fossils have been found, if not exact sites with maps and photographs. An ideal way to find new places to collect fossils is through mineral, fossil and archaeology clubs, and from high school or college earth science instructors. Often collectors are eager to share a few of "their" sites with others who express a sincere interest in paleontology.
Mesa Southwest Museum is big on Arizona's Fossil Record 
Geologic maps are similar to topo maps except they will often use colors to differentiate rock units. Often, fossil localities are printed on geologic and oil and gas investigation maps. An understanding of the local geology is immensely helpful in successfully predicting where fossils will be found, because certain rock units are more fossiliferous than others. The geologic maps show exactly where these units are exposed, so all that a collector must know is which units are fossiliferous. This is learned through experience or by research. Regardless, the geologic map is your crystal ball and tells you before you ever set foot in a region the age and type of rocks that occur, and thus indirectly whether fossils are likely to be found. Equally important, don't be afraid to write directly onto your map with circles, arrows, Xs, notations (Take the left or right fork in the road...rancher's wife loves cut flowers!) or even smiley faces. The memory is a funny thing when it comes to re-locating a favorite or important fossil locality.
There are some great maps online now and they're free and downloadable. An incredible Atlas of Arizona maps has just been made available. There's no excuse for you getting lost in Arizona...unless you happen to be me.

Steve Reynolds' Interactive Geological Map is incredible

Professional journals and guide books will help pinpoint areas of interest in terms of map coordinates. AS A SUCCESSFUL FOSSIL HUNTER, YOU NEED TO DO YOUR HOMEWORK BEFORE SETTING OUT. Don't think that taking a printout of this website will help you very much once you're in the middle of the desert or mesa country. It is very helpful in finding fossil sites to obtain maps which cover specific areas of interest. Topographic maps showing roads (both dirt and paved), arroyos, buildings, mines, and other landmarks, but more importantly hills and valleys, are usually time-saving aids in locating fossil collecting sites. These maps have been made for nearly all of Arizona, each map covers a small area (about 60 square miles) called a quadrangle. The indices are free and the individual maps can be ordered in person or by linking to the map and publication resources to the right.
Arizona Geological Survey
Arizona Geological Society
Bureau of Land Management
Arizona Department of Mines
Tucson's Map & Flag Center
Tucson Mineral and Gem World
Making Ready for the Collecting Trip
Before a fossil collector even goes on an outing there are some things to consider. On long trips, off paved roads in out-of-the-way places far from town, you must guard against becoming stranded. Vehicles should be in good shape with good tires, a spare, extra water and fan belts, boards and shovel (for when you get stuck), jack, and plenty of gas. Collectors should be in good physical shape and should not overexert themselves on the first outing. A volunteer of mine who had a jeep would carry three additional spare tires with him in the field at all times. I asked him, "Stan, why THREE spares?" He replied "because I can't hold FOUR!" Another bit of advice I leaned from Stan was to carry several cans of tire goop that you can put into a tire to fix a small leek until you get back to town.
That's me on the right. If you hate changing tires like I do, take along a special change of cloths and all you'll have to do is wait until some eager cowboy drives by. A quick get away is vital in such a scenario as you might find, after all of this, you may have bullet holes in all of your tires.
That's me ready to get my flat tire changed
Rattlers are frequently encountered in Arizona. 2,000 people are bitten each year, so a word of caution is advised. It makes sense when prospecting for fossils to have a snake bite kit for emergencies, and rigidly stick to the principle when out in this desert "Don't put your hand anywhere you can't clearly see."
That said, the vast majority of people who enjoy Arizona's outback will never have a bad experience with a rattlesnake, and most who are bitten by them do so by teasing shakes or attempting to collect them as souvenir pets to take home. Simply be aware that this is snake country and be "snake smart." Please don't kill any of Arizona's wildlife, especially snakes, as there are many more close by than just the one you see and lopping the head off of one makes neither moral or logical sense. It is their home, respect that.
Heavy, high boots are a must to prevent sore feet and falls. A hard hat is also advisable around quarries and roadcuts where rocks fall. Goggles or other eye protection will prevent blindness from flying chips of rock or metal which result from striking rocks. Protection from Arizona's hot sun can include a good hat, long sleeve shirt and/or sun screen. It is also a good idea to leave a note or tell somebody where you are going and always go with a companion. Note: Young children and older persons are most susceptible to hardships and weather extremes. Plan your trip accordingly.
Get permission from the owners or lessee to collect; respect protected public lands such as National, State or Local Parks where the removal of any natural object is prohibited. Indian reservations are essentially separate nations within the borders of the United States, and each has its own laws regarding trespass or fossil collecting without permits from the tribal government.
 A few years ago, among some great Mississippian plant specimens in an old coal mine in New Mexico , I uncovered a complete fossil dragonfly. Recently I visited this great fossil collecting site in the Sacramento Mountains (really great fossils!) only to find a new, 4,000 built right on top of the entrance. In hindsight, I could have bought that piece of land cheap a few years ago and preserved the locality.
  Fossil Collectors aren't always welcome
Keep vehicles off rangelands (possession of a four-wheel drive vehicle does not give you the right to travel off-road)
GUNS In the past I had recommended that all weapons be left at home. Unfortunately today, there has been an explosion in the numbers of illegal aliens crossing the US/Mexico border, and with that, a tremendous increase in crime, including assault and robbery. Arizona's law enforcement agencies not only allow citizens to carry firearms, many are encouraging it. In the p Having a pistol or other firearm on your possession is a deterrent, and has proven to greatly reduce the number of assaults and robberies. It is perfectly legal to carry a weapon such as a holstered pistol in most areas of Arizona (Among the few exceptions are bars, public governmental buildings or hospitals). In Arizona, it is also allowed and encouraged to carry a concealed weapon, but training from a certified instructor and a formal Concealed Firearm Permit is first required

Is it worth trying to rob me in the middle of the desert if you see this on my hip?

Watch cigarettes and fires. Much of Arizona is desert, and tinder dry much of the time. Wildfires are a common, dangerous and destructive occurrence in this state. Many caused by humans don't need to happen.

Pack out trash. There aren't many things worse than having to deal with someone else's trash in what could otherwise be a picture postcard setting.
Leave gates the way you found them, open or closed. This is more than just common courtesy. In the West, it's a BIG THING. Letting cattle out of a pasture or range, or letting unwanted livestock mingle with a herd is serious business and serves only to get a ranger angry, and can ruin opportunities for access to private land for others in the future. That said, a barbed wire fence isn't necessarily there to keep YOU out. Learn to climb fences safely, and expect to get your share of three-corner tares in your pants or skin.(ouch!) (bring some antiseptic ointment) . Learn to climb them quickly should an angry bull be on the other side. Better yet, learn to crawl under fences or take the time to find a gate. A locked gate is locked for a reason. Often the rancher will provide you with a key if you ask at the ranch house.
Rock hammer, chisels, knapsack, wrapping paper, toilet tissue and newspaper, shovel, pick (railroad or Marsh's), dental picks, glue (Paleobond), wisk broom, old tooth brush, maps (topographic, geologic and highway), food (sufficient for duration of trip and extra rations for emergency), spare tire (s), jack, heavy gloves, notebook and pencil (not ink, it runs!), water, sunhat, shake bite kit etc. Use logic and your imagination! If you have kids, bring some great snacks, and remember the little ones, physiologically, can't handle extremes in weather conditions that are nothing to an adult.
Basic necessary tools for field work. Eventually you'll need others