The best collecting sites are quarries, road cuts or natural cliffs, and outcrops. Open fields and level country are poor places to find rock exposures. Hills and steep slopes are better sites. Almost any exposure of rock provides some collection opportunities, but fresh, unweathered outcrops or manmade excavations offer the best locations. If possible, visit several exposures of the same rock to be sure a representative sample is selected.

Identifying Rocks
Many books about geology explain the identification and classification of rocks and describe the underlying geologic principles. Almost any recent general book on geology would help a rock collector. Geologic maps, which are useful guides for collecting, are also excellent identification aids. They show the distribution and extent of particular rock types or groups of rock types. Depending on size and scale, the maps may cover large or small areas. Most have brief descriptions of the rock types. Some are issued as separate publications; others are included in books.

Most geologic maps are issued by public or private scientific agencies. The most prolific publisher of geologic maps in the United States is the U.S. Geological Survey (USGS). "Geologic and Water-Supply Reports and Maps, (State)," a series of booklets published by the USGS, provides a ready reference to these publications for 13 States. The booklets also list libraries in the subject State where USGS reports and maps may be consulted. These booklets are available for less than $5.00 and may be obtained from: U.S. Geological Survey, Information Services, P .O. Box 25286, Denver, CO 80225

Rocks Tell the Story of the Earth
The Earth is made of rock, from the tallest mountains to the floor of the deepest ocean. Thousands of different types of rocks and minerals have been found on Earth. Most rocks at the Earth's surface are formed from only eight elements (oxygen, silicon, aluminum, iron, magnesium, calcium, potassium and sodium), but these elements are combined in a number of ways to make rocks that are very different. Rocks are continually changing. Wind and water wear them down and carry bits of rock away; the tiny particles accumulate in a lake or ocean and harden into rock again. The oldest rock that has ever been found is more than 3.9 billion years old, but rocks from the beginning of Earth's history have changed so much from their original form that they have become new kinds of rock. By studying how rocks form and change, scientists have built a solid understanding of the Earth we live on and its long history.

Types of Rocks
Geologists classify rocks in three groups, according to the major Earth processes that formed them. The three rock groups are igneous, sedimentary, and metamorphic rocks. Anyone who wishes to collect rocks should become familiar with the characteristics of these three rock groups. Knowing how a geologist classifies rocks is important if you want to transform a random group of rock specimens into a true collection.

Igneous rocks are formed from melted rock that has cooled and solidified. When rocks are buried deep within the Earth they melt because of the high pressure and temperature; the molten rock (called magma) can then flow upward or even be erupted from a volcano onto the Earth's surface. When magma cools slowly usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results.

A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock.

Sedimentary rocks are formed at the surface of the Earth, either in water or on land. They are layered in accumulations of segments - fragments of rocks, minerals, or animal or plant material. Temperatures and pressures are low at the Earth's surface, and sedimentary rocks show this fact by their appearance and the minerals they contain. Most sedimentary rocks become cemented together by minerals and chemicals or are held together by electrical attraction; some, however, remain loose and unconsolidated. The layers are normally parallel or nearly parallel to the Earth's surface; if they are at high angles to the surface or are twisted or broke, some kind of Earth movement has occurred since the rock was formed.

Sedimentary rocks are forming around us all the time. Sand and gravel on beaches or in river bars look like the sandstone and conglomerate they will become. Compacted and dried mud flats harden into shale. Scuba divers who have seen mud and shells settling on the floors of lagoons find it easy to understand how sedimentary rock forms. Sometimes sedimentary and igneous rocks are subjected to the pressures so intense or heat so high that they are completely changed. They become metamorphic rocks, which form while deeply buries within the Earth's crust. The processes of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks.

New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Some kinds of metamorphic rocks - granite gneiss and biotite schist are two examples - are strongly banded or foliated (foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.) Pressure or temperature can even change previously metamorphosed rocks into new types.