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A type of eroded topography that forms in semi-arid areas experiencing occasional periods of heavy rainfall. Sloping ground composed of sandstones and calcareous sediments underlain by clay or other soft materials is eroded over time into an intricate series of gullies and ravines. Different layers of rock weather at different rates, resulting in a variety of sculpted spurs and buttresses, as well as tall pillars of softer rock with a hard capstone.


A late- Proterozoic, early-Paleozoic continent that included ancient Europe (northern Europe without Ireland and Scotland). Baltica began moving toward North America in the Ordovician, starting the Taconic Orogeny. North America fully collided with Baltica in the Devonian, resulting in the Acadian Orogeny on the eastern edge of the continent.

banded iron formation

Rocks with regular, alternating, thin layers of iron oxides (e.g., hematite and magnetite) and either shale or silicate minerals (e.g. chert, jasper, and agate ). They are a primary source of iron ore.


A usually white, clear, or yellow mineral found in limestone, clay -rich rocks, and sandstones. Barite (BaSO4) occurs as flattened blades or in a circular pattern of crystals that looks like a flower and, when colored red by iron stains, is called a “desert rose.” Before federal laws were passed in 1906 to prevent the practice, finely ground barite was often added to flour and other foods to increase the weight.


A marine invertebrate animal belonging to the Infraclass Cirripedia in the Subphylum Crustacea, Phylum Arthropoda (thus related to crabs and lobsters), and characterized by a hard, calcareous shell attached to a hard surface.Barnacles are filter-feeders, resembling shrimps living upside down in a box. They are surrounded by calcareous plates that are attached to shells, such as the large Chesapecten shells found in Tertiary sediments of the Coastal Plain. It is sometimes possible to find fossil barnacles whole, often still attached to other shells.

barrier island

A long, thin island next to and parallel to a coastline.


An extrusive igneous rock, and the most common rock type on the surface of the Earth. It forms the upper surface of all oceanic plates, and is the principal rock of ocean/seafloor ridges, oceanic islands, and high-volume continental eruptions. Basalt is fine-grained and mostly dark-colored, although it often weathers to reds and browns because of its high iron content.
Basaltic magmas are produced by partial melting of the upper mantle. Materials melt when we increase their temperature, but a second way to melt a solid is to decrease the pressure. In the interior of the Earth this second mechanism—decompression—is far more important. When pressure on the mantle is released as it is forced up through the crust, it becomes basaltic magma.

basaltic andesite

A dark, fine-grained rock that is intermediate between basalt and andesite in silica content. Basaltic andesite is produced when the magmatic source of eruption is in transition between a deeper source, which tends to produce basalt, and a shallower source, which tends to produce andesite.
See also: andesite, basalt, magma, silica

basement rocks

The foundation that underlies the surface geology of an area, generally composed of igneous or metamorphic crystalline rock. In certain areas, basement rock is exposed at the surface because of uplift or erosion.


A large exposed structure of intrusive igneous rock that solidified at depth, and covers an area of over 100 square kilometers (40 square miles). While batholiths may appear uniform, they are actually composed of multiple plutons that converged to form one mass.


The topography of an underwater landscape.
See also: topography


A whitish, grayish, brown, yellow, or reddish-brown rock composed of hydrous aluminum oxides and aluminum hydroxides; the principal commercial source of aluminum.

Belt Supergroup

A 1.45-billion-year-old series of sedimentary rocks, found in the Northern Rocky Mountains, that contain sandstones and mudstones.
The Belt Supergroup is of particular note due to its age and excellent preservation. It is extremely rare that sedimentary rocks of over a billion years in age have not been warped, tilted, metamorphosed, or otherwise altered. The Belt Supergroup is also famous for its abundant and well-preserved stromatolites.


A clay, formed from decomposed volcanic ash, with a high content of the mineral montmorillonite.


A white, blue, yellow, green, or pink mineral, found in coarse granites and igneous rocks. It is a source of beryllium and used as a gemstone; the green variety is called emerald, the blue is known as aquamarine.


The number of kinds of organisms at any given time and place. Global changes in biodiversity through geologic time tells paleontologists that something is happening to the rate of extinction or the rate of origin of new species. Regional changes are influenced by migration, or the number of species supported by available food and space resources.


Carbon-based fuel produced from renewable sources of biomass such as plants and garbage. Energy is obtained through combustion, so greenhouse gases are still produced. Because plants get their carbon from the air, burning them for energy and re-releasing it into the air has less effect on climate than fossil fuels, whose carbon is otherwise sequestered away from the atmosphere.


A pile of lithified calcareous skeletal material formed on the sea floor from some variety of marine organisms, often including calcareous marine algae and marine invertebrates such as corals. Reefs are a form of bioherm in which organisms built the 3-dimensional structure, while banks are bioherms in which skeletal material accumulated through transport.


Organic material from one or more organisms.


The branch of geology that uses fossils to determine the relative age of sedimentary layers.


The organisms living in a given region, including plants, animals, fungi, protists, and bacteria.


The displacement of sediment and soil by animals or plants.


Any of various flammable mixtures of hydrocarbons and other substances, occurring naturally or obtained by distillation from coal or petroleum, that are a component of asphalt and tar and are used for surfacing roads and for waterproofing.

bituminous coal

A relatively soft coal containing a tar-like substance called bitumen, which is usually formed as a result of high pressure on lignite.


A marine or freshwater invertebrate animal belonging to the Class Bivalvia (or Pelecypoda) in the Phylum Mollusca. Bivalves are generally called “clams,” but they also include scallops, mussels, cockles, and oysters.
Bivalves are characterized by right and left calcareous shells (valves) joined by a hinge. Most are filter feeders, collecting food particles from the water with their gills.
During the Paleozoic, bivalves lived mostly on the surface of the ocean floor. In the Mesozoic, bivalves became extremely diverse and some evolved the ability to burrow into ocean floor sediments.


An extinct form of stemmed echinoderm, similar to crinoids. Blastoids possessed a nut-shaped body covered with interlocking plates, which was covered with fine hairlike structures for use in filter feeding. The body was held above the sea floor by a stalk of stacked disc-shaped plates.

body fossils

Fossils that consist of an actual part of an organism, such as a bone, shell, or leaf.


An extraterrestrial object of any composition that forms a large crater upon impact with the Earth. In astronomy, bolides are bright meteors (also known as fireballs) that explode as they pass through the Earth’s atmosphere.


A cold temperate region relating to or characteristic of the sub-Arctic climatic zone, often dominated by conifers, birch, and poplar.

boundary layer

The atmospheric layer closest to the surface of the Earth, which has a high relative humidity and is affected by the Earth’s heat and moisture.


A marine invertebrate animal belonging to the Phylum Brachiopoda, and characterized by upper and lower calcareous shell valves joined by a hinge, and a crown of tentacles (lophophore) used for filter feeding and respiration. Brachiopods are the most common fossil in Paleozoic sedimentary rocks.
Brachiopods look somewhat similar to the clams that you find at the beach today. Brachiopods and bivalves both have a pair of hinged shells (valves) to protect themselves while feeding. However, the soft parts of modern brachiopods tell us that they are completely unrelated to bivalves. Brachiopods have a special structure formed by tissue with thousands of tiny hair-like tentacles stretched along a coiled piece of internal shell material. These tentacles catch and move small particles toward the mouth. This body plan is very different from that of bivalves, which have a larger, fleshy body and collect particles with their gills.
To tell the difference between a brachiopod and a bivalve, look for symmetry on the surface of the shell. Bivalve valves are of equal size and mirror image shapes. Brachiopods’ bottom valves, however, are slightly bigger and often have a different shape.

braided stream

A stream consisting of multiple, small, shallow channels that divide and recombine numerous times, forming a pattern resembling strands of braided hair. A braided stream carries more sediment than a typical stream, causing the formation of sandbars and a network of crisscrossing streams.


A pyroclastic rock composed of volcanic fragments from an explosive eruption.


See hydrothermal solution

British Thermal Unit (BTU or Btu)

The most commonly used unit for heat energy. One Btu is approximately the amount of heat required to raise one pound of water by one degree Fahrenheit. A Btu is also about the amount of energy released by burning a single wooden match.


A liquid chemical element (Br), with corrosive and toxic properties. It is commonly used in pesticides, flame retardants, and as a gasoline additive. Bromine is highly soluble, and it is the fifth most abundant element dissolved in ocean water. It can be extracted from brines, some of which are associated with salt deposits.


A marine or freshwater, colonial invertebrate animal belonging to the Phylum Bryozoa, and characterized by an encrusting or branching calcareous skeleton from which multiple individuals (zooids) extend from small pores to filter feed using crowns of tentacles (lophophores).
Bryozoans have a long and exemplary fossil record. One of the more common Paleozoic varieties looks like fine-mesh cloth with numerous tiny holes in which the individual animals in the colony lived. Although they function somewhat like coral, and are often found in similar environments, bryozoans are more closely related to brachiopods.


An isolated hill with steep, often vertical sides and a small, relatively flat top.