Descriptionary

allochthonous referring to a geological object cre­ated in a location other than where it was found, as a glacial erratic.

alluvial fan deposits of alluvial material, such as rock and silt, that fan out and form an apron at a mountain's base or lower slope; the land counterpart of a river delta.

alluvium any deposits of sand, mud, or silt created by moving water.

amber tree resin that has turned to stone, the inte­rior of which may contain preserved insects or other small, prehistoric items of interest.

anticline strata of rock bent into an arch, the reverse of a syncline.

aquifer an underground pocket of rocks, gravel, or other permeable material through which groundwater flows.

archipelago a group or scattering of related islands.

arroyo a dried-up stream channel often found in desert areas. Also known as a wadi or wash.

asthenosphere the zone between 50 and 250 kilo­meters below the surface of the Earth where rock is heated and pressurized sufficiently to flow; respon­sible for many of the Earth's vertical and horizontal movements.

astrogeology geology of celestial bodies.

Atlantic Ridge a mountain range under the mid- Atlantic Ocean extending from Iceland to Antarctica.

autochthonous referring to a geological object cre­ated in the location where it is found.

badland a desert area of eroded ridges, peaks, and mesas.

bajada a series of coalescing alluvial fans at the base of a mountain or mountains.

basin an expansive depression in the Earth, which fills with deposits of sediment.

basin and range topography any broad expanse of land characterized by fault-bounded mountains and sediment-filled valleys.

batholith a huge igneous mass with a surface area of at least 100 square kilometers and increasing in size as it extends downward, intruded into other rock and found under or within mountain ranges. Also known as a pluton.

bedding the layers in sedimentary rock.

bedding plane the surface area separating one deposit of sedimentary rock from another of different character.

bedrock any rock layer underlying soil or sediment. B-horizon zone of soil below the A-horizon.

biostratigraphy the science of determining the ages of rock layers and how they were distributed or deposited by the study of the fossils they contain.

bolson a flat desert valley surrounded by mountains that drains into a shallow central lake.

Burgess shale world-famous, 505-million-year-old shale formation in Canada, renowned for its high- quality fossils of bizarre, prehistoric creatures.

butte a mesa that has eroded so that its width and length are less than its height. Also known as a monu­ment.

calcareous containing calcium carbonate, formed from the shells of bivalves.

Cambrian explosion a broad, evolutionary expan­sion of new animal types and complexities that, according to the fossil record, appeared mysteriously 530 million years ago.

carbon 14 a radioactive isotope of carbon that has a half-life of 5,730 years. Used to date objects or events up to 50,000 years ago.

carbonate a mineral made up largely of calcium carbonate, from seashells and coral formations.

Carboniferous period a geological period from 359.2 million to 299 million years ago, named for the widespread beds of coal laid down at the time.

cast a fossilized replica of an organic object, formed when sediment fills a mold of the object.

compaction any fossil that has been flattened, either by the weight of an overlaying of rock or by a glacier.

compression a trace of fossilized organic matter, the remains of a crushed organism.

concretion a mass of rock, often egglike or spheri­cal in shape, and formed by accumulation of succes­sive layers of sediment around a leaf, shell, fossil, or other object that serves as a nucleus, and is usually broken open by fossil collectors in search of such artifacts. Concretions are sometimes mistaken for fos­silized dinosaur eggs.

continental drift the drifting of the continents due to spreading of the seafloor.

continental shelf a sloping shelf of a continent that extends into the ocean then descends sharply.

coprolite fossilized animal feces.

core the inner portion of the Earth, beginning approximately 2,900 kilometers down.

craton the large, generally immobile center portion of a continent.

creep the gradual sliding or slipping of soil and sur­face material down a slope.

crust the outermost shell of the Earth, extending about 35 kilometers down.

deflation wind erosion of unconsolidated material.

delta the large, delta-shaped deposit of silt found at the mouth of rivers.

dendrite appearing on some rocks, a tiny branching line made up of iron and manganese oxides, evidence of water filtration.