Sediment ranges in size from large boulders to very fine mud. Sediments coarser than two millimeters are called gravel. Sand is defined as sediment with sizes between one-sixteenth of a millimeter and two millimeters. All sediment finer than sand is called mud. The coarser part of the mud is called silt, and the finer part is called clay.

Living things are made up of chemical compounds, most of which are organic compounds. Organic compounds consist mainly of carbon, oxygen, and hydrogen. After a plant or animal dies, it decomposes. As organisms decompose, their organic compounds change into simpler compounds like carbon dioxide and water. Decomposition is fastest when the organisms are in water that contains dissolved oxygen. Organisms can also decompose even without oxygen. Some kinds of bacteria feed on plant and animal tissues even though there is no oxygen. These are called anaerobic ("no air") bacteria.

A fossil is any evidence of past life. Fossils formed from animal bodies or their imprints are called body fossils. When people think about fossils, they usually think about body fossils. Trace fossils are another kind of fossil. A trace fossil is any evidence of the life activity of an animal that lived in the past. Burrows, tracks, trails, feeding marks, and resting marks are all examples of trace fossils. It is usually hard to figure out exactly which kind of animal made a particular trace fossil.

Two important things determine the Earth's climate. One is the amount of heat the sun delivers to the Earth. Also important is where the Earth's continents are located relative to the equator. Continental ice sheets cannot develop unless one or more continents are located at high latitudes.

Thin cores of ice, thousands of meters deep, have been drilled in the ice sheets of Greenland and Antarctica. They are preserved in special cold-storage rooms for study. Glacier ice is formed as each year's snow is compacted under the weight of the snows of later years. Light bands correspond to the relatively fresh, clean snows that fall in the summer when warmer conditions bring more moisture and precipitation. Dark bands mark the polar winter season, when little new snow falls on these frigid deserts and blowing snow is mixed with dust, discoloring the white snow.

The Earth's climate has changed greatly through geologic time, and even in recent centuries. The study of past climates is called paleoclimatology ("paleo-" means "early" or "past").

It is easy to understand how climate can vary over very large areas, because of slight changes in temperature or rainfall. Climates can also vary over very short distances. Local differences in climate are described by the term "microclimate." Low-lying areas are often colder at night than higher ground nearby. On clear nights, the ground is chilled as its heat is radiated out to space. The cold ground then chills the air near the ground. The chilled air is slightly denser than the overlying air, so it tends to flow slowly downhill, in the same way that water flows downhill.

The plant community in an area is the most sensitive indicator of climate. Areas with moderate to high temperatures and abundant rainfall throughout the year are heavily forested (unless humans have cleared the land for agriculture!). Areas with somewhat less rainfall are mainly grasslands, which are called prairies in North America. Humans have converted grasslands into rich agricultural areas around the world. Even in areas with high yearly rainfall, trees are scarce if there is not much rainfall during the warm growing season.