absolute age

age measured in years and is usually determined by measuring isotopic decay


a mountain building period from 420 to 380 Ma

accommodation space

the space that is available for the deposition of sediments. In river systems, changes in accommodation are controlled by the river gradient, water discharge rates, and sediment supply.

accretionary prism

a mass of sedimentary material scraped off of oceanic crust during subduction that piles up at the edge of the overriding plate


also spelled eolian, wind processes that shape the surface of the Earth (or other planets)


growth either vertically or horizontally of morphological features. In coastal areas, these include the beach, dunes, sand banks, tidal flats, and salt marshes


a mountain building period from 350 to 250 Ma

alluvial fan

a landform shaped like a gentle mound or cone with its apex at the mouth of its sedimentary source, typically a mountain stream.


unconsolidated detritus (clay, silt, sand, and gravel) deposited by streams and floods


a cephalopod (similar to a nautilus) with a sutured shell; commonly it grows in a spiral form. They lived in the Jurassic and Cretaceous and are excellent for biostratigraphy and determining relative age

angle of repose

the maximum angle that a soil, sediment, or other loose material can be placed or accumulate and not move down-slope. This angle varies for different types of material and moisture conditions


an anhydrous calcium sulfate mineral (CaSO4) that is formed in deeply buried evaporite deposits.  It absorbs water in near-surface environments to form gypsum.


characterized by a lack of oxygen; thus, they do not support life such as burrowing or benthic organisms


A geologic feature which is composed of layers with a convex form


a body of rock and/or sediment that holds groundwater


a linear belt of volcanoes that forms where an oceanic plate collides with the either a continental or oceanic plate and subducts beneath it


a sedimentary rock consisting of sand or gravel with both feldspar and quartz; typically created by rapid weathering of granite

assemblage (of fossils)

a suite of fossils collected from a rock unit. These are assumed to have lived at the same time and environmental conditions

asymmetrical ripple marks

a sedimentary structure produced by fluid flow in one direction in which the limbs of the ripple are at about the different angles

back arc basin

a basin formed near a volcanic arc on the side opposite the oceanic trench; this is the result of extension caused by mantle convection behind a subduction zone and its island arc

back reef

the area behind a reef that may include shallow marine sediments, shoreline, and evaporite sedimentary environments


a part of the system of dunes that is furthest from the ocean


the beach between the foreshore and coastline. The backshore is typically dry and does not have vegetation and often characterized by berms or dunes. The backshore is only exposed to waves under extreme events with high tide and storm surge


the northern craton of Europe underlying much of Scandinavia that was an independent continent that collided with northeastern North America in the early Paleozoic to form the Taconic, or Caledonian Mountains


a landscape that lacks plant or animal life

barrier island

a coastal landform that is a barrier between the ocean and mainland generally running parallel to the main coastline. They typically occur in a chain and extend from several to 100's of kms

Barrovian sequence

a series of metamorphic zones with both pressure and temperature increase generally due to continental collision


a topographic or bathymetric low area. Structural basins have dipping strata toward the center. Depositional (sedimentary) basins have thick accumulations of sediment compared to adjacent regions


the depth of water in oceans, seas, or lakes


area of relatively shallow water in a coastal environment, separated from the open marine conditions by a natural barrier (a sand spit, a barrier island or a coral reef), but with an access to the sea


an area of unconsolidated material that extends from low-tide line to an area with permanent vegetation

bed forms

the ocean bottom is seldom flat and often undulates from the interaction of currents and waves. These include ripples and sand ridges


distinct layers of sediment that have a thickness greater than 1 cm


related to the seafloor. Benthic organisms live on or in the substrate of the ocean floor


a clay-rich rock made of smectite that formed from alteration of volcanic ash. It is mined as a major constituent of drilling mud.

bilateral symmetry

an object that can be divided into two symmetrical parts across a unique plane, sometimes called a mirror plane


the branch of stratigraphy that uses fossils to determine relative age and correlate successions of sedimentary rocks


reworking of soil and sediment by animals or plants

bottom simulating reflector

a seismic reflection occurring in the upper few hundred meters of marine sediments mimicking the seafloor

braided river

consists of a network of river channels that split and join. The channels can be separated by small, often temporary, islands.


a type of aquatic invertebrate animals. They are filter feeders that extract food particles out of the water using tentacles.


process in which sedimentary layers are piled up and the sediments beneath are buried, sometimes by hundreds of meters of sediment above


a carbonate mineral, CaCO3. This is the stable polymorph at the Earth's surface


a white to reddish brown material ranging from soil to rock produced in a soil in semiarid to arid environments. Caliche can have sand or gravel zones that are cemented by calcium carbonate.


the first period in the Paleozoic era, between the Precambrian eon and the Ordovician period (541 to 485 Ma)


process of transforming clastic sediments into rocks by the precipitation of mineral matter in their pore spaces

chemical sedimentary rock

form when minerals in solution become supersaturated and inorganically precipitate


a principal taxonomic category that ranks above order and below phylum


a fragment of geological detritus broken off other rocks by physical weathering


sedimentary rocks composed of broken pieces of older rocks

clastic rock

composed of fragments or clasts, of geologic detritus including both minerals and rocks


these sedimentary layers are timelines that represent a moment in geological time


a strip of land from the coastline inland to an area that is not influenced by coastal processes

coastal area

both the land and the sea bordering a shoreline


of the amount of electrical current a rock can carry or its ability to carry a current


parallel strata that have a similar geologic history and were deposited in succession without interruption


a boundary which separates one rock body from another. These can be depositional, unconformable, and intrusive contacts

continental drift

the hypothesis proposed by Alfred Wegener in 1915 that stated the continents were moving

continental shelf

the submerged portion of continent crust in relatively shallow water. It extends from the shoreline to the drop-off point marked by the shelf break.


a plate boundary where two plates move towards each other


process of relating either characteristics, time or stratigraphic position of separated rock units. This can be lithologic (similar rock types), biostratigraphic (similar fossil content), chronostratigraphic (similar ages), or chemostratigraphic (similar chemical signatures)

costa (plural = costae)

Rib or rib-like structures


a geological period from about 145 to 66 Ma. It is the third and final period of the Mesozoic Era, as well as the longest. The name is derived from the Latin creta, chalk


groups of inclined layers and sloping layers are known as cross strata. Cross-bedding forms on a sloping surface such as ripple marks and dunes

crown (crinoid)

a cup-like central body with a set of five rays or arms, usually branched and feathery


a low, flat area near the mouth of a river typically with a triangular shape that is crossed by distributaries of the main river. It also includes the shoreline and off shore environments related to the delta plain. These form because the sediment load supplied by the river is too high to be washed away. They are classified by the main source of energy that supply the sediment load such as wave dominated, fluvial (river) dominated, or tide dominated.


an area of maximum deposition in a stratigraphic unit


the fourth period of the Paleozoic era, between the Silurian and Carboniferous periods (405 to 345 Ma)

diapir (salt)

a mass of salt that has flowed ductilely and has discordant contacts with the rocks above and on its sides


a linear feature where two plate boundaries move apart from each other

drainage basin

the area of land in which runoff feeds into a stream, lake, or reservoir. A first order drainage basin feeds directly into the ocean


there are two types of dunes: subaerial and subaqueous. Most only know about the subaerial dunes from trips to the beach and/or a desert. They are made of wind-blown sand and typically are vegetated. Subaqueous dunes are also known as sandwaves.


a metamorphic rock composed of garnet and pyroxene. Also, a facies of metamorphism formed at high pressure and temperature.

Ediacaran Period

the uppermost division of the Proterozoic Eon of Precambrian time and latest of the three periods of the Neoproterozoic Era, extending from approximately 635 million to 541 million years ago. Also called Vendian Period


sedimentary features formed by wind including sand dunes and silt (loess) deposits


longest division of geological time, subdivided into eras


unit of time that is a component of a period, such as Eocene


a unit of time such as Mesoproterozoic, Neoproterozoic, Paleozoic, Mesozoic, and Cenozoic. Eras are divided into periods and epochs.

Euler pole

a mathematical pole of rotation that describes the relative movement of two or more plates on a sphere


worldwide changes in sea level due to changes in volume of water in the oceans versus ice sheets


a type of sediment that consists of water-soluble minerals deposited after concentration and crystallization by evaporation from an aqueous solution


igneous rocks that form when magma cools at or near the Earth's surface


for sedimentary rocks, it is a body of rock with specified characteristics such as type of sediment


a principal taxonomic category that ranks above genus and below order


a triangular shaped sedimentary deposit, often called an alluvial fan when it is composed of alluvium or overwash fan when it occurs in a coastal environment

fan delta

a type of alluvial fan that flows into lowlands that is reworked by shoreline and gravity processes


a fracture or zone of fractures between two pieces of rock


an igneous rock rich in feldspar (fel) and quartz (si)


planar arrangement of structural or textural features in metamorphic rocks along straight or wavy planes


the part of a system of dunes closest to the ocean

foreland basin

structural basin that develops adjacent and parallel to a mountain belt formed from the mass created by crustal thickening that causes the lithosphere to bend creating accommodation space


this is the zone between low-tide line and high-tide line, also called the beach face. It is typically wet from the tides and waves and can be destroyed by storm surge.


a fundamental rock unit based on similar characteristics in lithology, including characteristics as color, mineralogy, and grain size. Formations may represent deposition over short or long time intervals, may be composed of materials from several sources, and may include breaks in deposition. Formations are named after geographic features where they were first studied.

gamma radiation

gamma (γ) rays come from the radioactive decay of naturally occurring radionuclides


a principal taxonomic category that ranks above species and below family (plural is genera)

geologic time scale

a system that geologists use to relate chronological dating to geological strata (stratigraphy)


a device that converts ground movement (velocity) into voltage, which is recorded as a seismic response and is analyzed for structure of the earth.

geothermal gradient

change in temperature with respect to increasing depth. Away from the edges of lithospheric plates, it is about 25–30 °C/km


a metamorphic rock made up of bands that differ in color and composition; generally there are light-colored bands rich in feldspar and quartz and dark-colored bands rich in mafic minerals such as biotite or amphibole


graben and horst refer to regions between normal faults and are either higher (horst) or lower (graben) than the area beyond the faults

graded beds

a sediment consisting of a mixture of coarse and fine detrital material


a coarse-grained, intrusive igneous rock typically composed of quartz, feldspars, and amphibole/biotite.


measure of the resistance to scratching by another object or mineral


hornfels are formed during contact metamorphism or the baking by heat of intrusive igneous masses. These are often nonfoliated or massive and in some cases very durable


linear volcanic regions that are probably fed by underlying mantle that is anomalously hot compared with the surrounding mantle


area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust


an assemblage of trace fossils that provide an indication of the paleoenvironment that these organisms inhabited

igneous dike

a sheetlike igneous body that is often oriented vertically or steeply inclined to the bedding or layering


the area between the foredune and backdune


an igneous rock that forms when magmas cool beneath the Earth's surface


an animal lacking a backbone such as a clam or worm

irregular echinoid

these have a definite front and back and move in a particular direction


the principle that Earth's crust is floating on its mantle


a geologic period that spanned 56 million years from the end of the Triassic Period 201.3 Ma to the beginning of the Cretaceous Period 145 Ma. This constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles


a relatively shallow, narrow body of water that is parallel to the shoreline between a barrier bar or island and the mainland. It is connected to the ocean by narrow passes.  It may have high salinity and significant tides.


distinct layers of sediment that have a thickness less than 1 cm


visible features of an area of land and its landforms

large igneous province

an extensive area of predominantly iron- and magnesium-rich (mafic) rock that form by processes other than normal seafloor spreading; they are the dominant form of near-surface magmatism on the terrestrial planets and moons of our solar system


a soil rich in iron oxide and alumina, poor in silica, and depleted in alkali elements.  It forms by intense tropical weathering.


a continental block or craton consisting of most of North America and Greenland that has retained its integrity since late Proterozoic time. Since the collision of Baltica, the resulting continent is Laurasia.


a remote sensing method that uses pulsed laser light to measure distances to the Earth


process where loose grains of sediment are converted into rock, often by cementation


the general characteristics of rocks and rock types present in a stratigraphic division of earth


the uppermost layer of the Earth as defined by physical properties. It is composed of the crust and uppermost, rigid part of the mantle.


a sub-discipline of stratigraphy focuses on strata or rock layers


sometimes called well logging. Methods to identify characteristics of subsurface formations either by samples of physical responses (geophysical well logging) that are displayed as a function of depth of a borehole.

longshore current

the dominant current in the nearshore zone; typically runs parallel to the shore


an interval of time when relative sea level is low and the continental shelf and carbonate platforms are exposed. Extensive deltas form at the margin and a lot of sediment can be transported into deep marine basins.


how light interacts with the surface of a mineral


an igneous rock that is rich in magnesium (ma) and iron (f), often has abundant pyroxene, olivine and amphibole


molten matter beneath or within the Earth's crust from which igneous rocks are formed


a branch of stratigraphy in which stratigraphic divisions are made on the basis of changes in magnetic signals


a layer in the Earth or a planet that is between the core and crust


sedimentary material that is a mixture of carbonate and clay; both present in subequal amounts


in coastal areas, these are low-lying and often inundated by tidal waters. They tend to be heavily vegetated and low-energy sedimentary environments.


the tendency to scour the outer bend of a river channel and deposit sediment on the inner bend


a formal stratigraphic unit that is a part of a formation; these need not be mappable

Mercator projections

a map projection of the world onto a cylinder so that all the latitude lines have the same length as the equator


interval of geological time from 252 to 66 million years ago. Some call it the Age of Reptiles or the Age of Conifers. The Mesozoic is one of three geologic eras of the Phanerozoic Eon, between the Paleozoic and the Cenozoic

metamorphic rocks

the process by which heat, pressure, or fluids transform the minerals and textures of the rock

mid-ocean ridge

an underwater mountain and rift system formed by an oceanic divergent plate boundary


heterogeneous rock with properties of both igneous and metamorphic rocks; often formed by partial melting


a naturally occurring, inorganic, solid that can be defined by a chemical formula and a crystal structure.


a small basin typically formed by subsidence in thick salt

mountain belts

an aligned group of mountain ranges that form from the same cause, usually an orogeny or convergent plate boundary


fine grained sediment with a high concentration of clay minerals

natural gas

a mixture of hydrocarbons generally with 1-5 carbon atoms with variable amounts of hydrogen atoms that is gaseous at the surface produced by degradation of organic matter in sedimentary rocks and microbial activity


this zone extends from the low-tide line to beyond where waves influence the sedimentation (sometimes called the breaker zone).


a organism that actively swims in the water column, compare with benthic and planktonic


a sedimentary rock composed of finely crystalline quartz that may form from thermal alteration of bedded chert


this is not well defined, but is a zone beyond the nearshore. Typically it begins beyond where nearshore currents and waves affect the water column.


in geology: the advance of a sea beyond its former shore

in geophysics: pattern of reflections in seismic data that occur during periods of transgression (sea level rise)


a type of limestone made of sand-size, well-rounded carbonate particles (oolith) with concentric layers formed in a shallow, wave agitated environment that are cemented together


a section of oceanic crust and its underlying upper mantle that has been uplifted and exposed, generally now on continental crust


a principal taxonomic category that ranks above family and below order


second period of the Paleozoic era, between the Cambrian (~486 Ma) and Silurian (~444 Ma) periods

organic sedimentary rocks

form from the accumulation and lithification of organic matter, such as leaves and other plant or animal material


a mountain-building event typically resulting from a convergent tectonic boundary


caused by waves that rush over a natural or artificial coastal barrier. This term can also be used for the sand deposit on the leeside of the barrier. These typically form during storm events.


the climate during some past geological time


an environment that has been preserved in the rock record at some time in the past


the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from 541 to 252 million years ago


a supercontinent that existed from ~300 to 200 Ma and included almost all of Earth's continental crust


a super ocean that surrounded all of the supercontinent Pangea

patch reef

a small carbonate reef that is moundlike and separated from other patch reefs or bioherms


pertaining to the open ocean, often referring to organisms that do not need seafloor or continental environments


a unit of time such as the Cretaceous; a component of an era and composed of epochs


the ability for fluids (including gas) to flow through rocks


a geologic period that spans 47 million years at the end of the Paleozoic era, from the end of the Carboniferous period (298.9 Ma) to the beginning of the Mesozoic era with Triassic period (251.902 Ma)

petroleum system

involves deposition, generation, migration, entrapment, and preservation of hydrocarbons in sedimentary basins


the current geologic eon in the geologic time scale. This has the abundant animal and plant life. It covers 541 million years to the present, and it began with the Cambrian Period


a principal taxonomic category that ranks above class and below kingdom


a diverse variety of aquatic organisms that are cannot swim against a currents, tides, and waves. A common group of plankton are foraminifera


an organism that drifts in the water column, compare with nektonic and benthic

plate boundaries

the edge of either a continental or oceanic tectonic plate


a large flat area that covers a large region and is elevated above its surroundings. It is larger than a mesa.


a dry, vegetation free flat area within a desert. It can be filled with ephemeral lakes (playa lakes) during wet seasons that evaporate during dry seasons


two or more minerals that have the same chemical composition but differ in their internal atomic arrangement and crystal structure


a solid material that exists in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including minerals, polymers, and metals


the amount of void space in a rock


a large mineral in a metamorphic rock which has grown in the fine grained matrix


seaward movement of a shoreline and coastal area, occurs when the rate of sediment supply is more than the rate of relative subsidence


original, unmetamorphosed rock from which a metamorphic rock was formed; sometimes called the parent rock

pull-apart basin

a basin formed between two adjacent strike-slip faults in a region where subsidence generates accommodation space for the deposition of sediments


a vesicular felsic igneous extrusive rock with very low density such that it floats in water


metallic, yellow mineral made of FeS2 and typically occurring as intersecting cubes. Often called Fool's Gold

quadrangle (quad)

a topographic map sheet published by the United States Geological Survey with boundaries defined by lines of longitude and latitude


mineral consisting of SiO2, silicon dioxide, found in all rock types, the stable polymorph at the Earth's surface

quartz arenite

a type of sandstone with >95% quartz. This high quartz content is a result of quartz that has been "recycled" during several depositional cycles


a metamorphic rock with a sandstone protolith


the current and most recent of the three periods of the Cenozoic Era from 2.588 ± 0.005 million years ago to the present

radial symmetry

symmetry around a central axis


emission of energetic particles and/or radiation during radioactive decay

radioactive decay

spontaneous emission of particles (alpha or beta) and gamma rays from the nucleus of an unstable nuclide. The resulting nucleus may be stable or unstable. If it is unstable, decay continues until there is a stable nucleus

radiocarbon dating

a method for determining absolute age by measuring the amount of carbon-14 remaining in organic material. Only used on rocks less than 70,000 years


a mound or ridge built by calcite-secreting organisms such as coral, rudist mollusks, or sponges. They are resistant to waves and stand above the surrounding seafloor.


another term for reflection coefficient or how much energy is reflected by the stratigraphic units


a seismic reflector may represent a change in lithology, a fault, or an unconformity


a geological process occurring when areas of previously submerged seafloor are then exposed above sea level

regular echinoids

these have no front or back end and can move in any direction

relative age

geologic age of an organism, rock, or event that is defined in relation to other rocks or events rather than in years before present or an absolute age


a rock property which represents how strongly it opposes the flow of electric current

restricted shelf

a margin that has restricted flow of seawater, often associated with evaporites and arid conditions


landward retreat of a shoreline that occurs when the rate of sedimentation is slower than the rate of subsidence

rock cycle

a concept in geology that describes transitions among the three main rock types: sedimentary, metamorphic, and igneous


a supercontinent formed in the Proterozoic eon (1000 to 700 million years ago)

root structure

a thin, tubular, near-vertical, and sometimes downward-branching sedimentary structure formed by the filling of a hole left by a root, also called a root cast


a bivalve mollusk that existed from Late Jurassic to Cretaceous. The two valves (shells) were very unequal. This grew as solitary animals or in reefs.

sag basin

a basin formed by extension of continental crust typically containing lake deposits and evaporites

salt sheet

a salt mass that is longer and wider than its thickness. It is fed by a salt diapir.

salt tectonics

structural features caused by the movement (flow) of salt under geologic stresses. This can include salt domes and canopies and can also effect overlying sedimentary strata


a clastic sedimentary rock composed of sand-sized minerals, rock fragments, or organic material


a vesicular mafic igneous extrusive rock which often forms in cinder cones

sea level rise

a rise of the mean sea level; this causes coastline retreat and increased flooding

seismic character

a set of characteristics with similar seismic reflections including bedforms, their geometry, lateral continuity, amplitude, frequency, and interval velocity


a wall that divides a cavity into smaller sections

septae (corals)

radial internal walls within the chamber of a coral that support their soft tissue


indurated mudstone, usually clay rich, with bedding planes lined with clay which produce fissility (easy separation of the layers)


the flat part of the continental margin between the shoreline and the shelf margin. Generally it is less than 180 m (600 ft) water depth. They can also occur in intracratonic basins


the third period of the Paleozoic era, between the Ordovician and Devonian periods (444 to 416 Ma)


a tube running between the chambers in cephalopods that allows them to adjust their internal pressure and buoyancy

source rock

a term for hydrocarbons that refers to a sedimentary rock that is rich in organic matter and has been subjected to heat that will generate oil or gas


a group of organisms that can reproduce with one another and produce fertile offspring

spherical symmetry

a body shaped like a sphere arranged around a central point


a siliceous or calcareous support that stiffens and supports tissues of various invertebrates, especially siliceous sponges


multicellular organisms with bodies full of pores allowing water to circulate through them. They are made of collagen and some have silica-rich spicules.

starved basin

a sedimentary basin in which the rate of sedimentation is less than the rate of subsidence

storm surge

a sudden rise in sea level in coastal areas during a storm or hurricane caused by strong winds blowing onshore


tabular or sheet-like layers of sedimentary rock. A single layer is called stratum.


layering in sedimentary rocks and igneous rocks formed at the Earth's surface such as lava flows and volcanic fragmental deposits. These layers can range from several millimetres to many metres in thickness and vary greatly in shape

stratigraphic column

a representation used to describe the vertical sequence of rock units

stratigraphic section

a sequence of rock layers or formations in the order they were deposited


a branch of geology that focuses on the study of rock layers and layering. It is can cover both sedimentary and layered volcanic rocks. Stratigraphy has two related subfields: lithostratigraphy and biostratigraphy


color of a mineral in a powdered form


a process that occurs at convergent plate boundaries where one plate descends beneath another into Earth's mantle.


downward motion of land; often related to soil compaction, extraction of natural gas or other mineral resources, and ground and water withdrawal. It can also occur with earthquakes, glacial isostatic adjustment, erosion, sinkhole formation, or sediment loading


a large landmass (notably Pangaea, Gondwana, and Laurasia) that then divided to form the present continents


the zone where two plates meet during continent-continent collision

suture pattern

a feature used to classify cephalopods with patterns such as ammonite, ceratite, goniatite, and nautiloid


the zone where two continental plates meet during convergence

swelling clay

smectite or vermiculite clay minerals that swell by absorbing water

symmetrical ripple marks

a sedimentary structure produced by the back-and-forth motion of waves or tides in which both limbs of the ripple are at about the same angle


A downward closing fold


a mountain building period from 550 to 440 million years ago


coarse, angular rock fragments formed by falling or sliding rocks down a slope

ternary plots

a graph that depicts the ratios of the three variables as positions in an equilateral triangle

Tethys Ocean

an ocean that occupied the area along the Alpine to Himalayan mountain belt in the Jurassic. Named after Tethys, the wife of Oceanus in Greek mythology

trace fossil

a fossil that records biological activity but not the preserved remains of the plant or animal


a plate boundary where two plate boundaries move horizontally beside each other


a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding


a topographic depression of the sea floor, relatively narrow in width, but very long usually associated with convergent plate boundaries


a geologic period that spans 50.6 million years from the end of the Permian Period 251.9 Ma, to the beginning of the Jurassic Period 201.3 Ma. This is the first and shortest period of the Mesozoic Era


a biome where tree growth is shortened by low temperatures and short growing seasons


sedimentary deposits on the sea-bottom formed by massive slope failures that result in currents carrying grains sorted by density

turbidity currents

the currents that deposit grains sorted by density

type locality

the geographic locality of site chosen for the description of a geologic unit. Can also be used for the site that a fossil or mineral is first discovered

type section

similar to a type locality, this consists of a representative series of rock units. Once described it can never be changed

U-Pb geochronology

a method of determining absolute age using isotopes of  uranium, thorium, and lead typically using the mineral zircon. Yields ages between 4,500 to 5 Ma


a buried erosional or non-depositional surface separating two rocks or strata of different ages, indicating that sediment deposition was not continuous


the theory that the same natural laws and processes that operate in our present-day scientific observations have always operated in the universe in the past and apply everywhere in the universe. It is sometimes referenced as "The present is the key to the past".


shells of animals especially paired shells of brachiopods and bivalves such as clams and oysters


paired light and dark sedimentary layers which may indicate cyclical seasons such as summer and winter


an animal with a backbone such a dog or cat

volcanic ash

fragments of rock, minerals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm in diameter

volcanic material

larger fragments of rock, minerals, and volcanic glass created during volcanic eruptions and measuring more than 2 mm in diameter

Walther’s Law

this theory states that the vertical succession of facies reflects lateral changes in environment of deposition

wavy bedding

occurs when mud is deposited over the entire bed of rippled and/or cross stratified sand following the alternating concave-convex nature of the ripples creating a wavy appearance

way-up indicators

a characteristic seen in a sedimentary or volcanic rock that makes it possible to determine whether they are still the right way up (i.e. in the attitude in which they were originally deposited)


in structural geology, it is an eroded area that shows rocks beneath a thrust sheet


a zirconium silicate mineral, ZrSiO4, that is present in felsic and intermediate igneous rocks as well as metamorphic and sedimentary rocks.  Thus, it is invaluable for U-Pb geochronology


a single animal that is part of a colony


small and microscopic organisms drifting or floating in the sea or fresh water, consisting chiefly of diatoms, protozoans, small crustaceans, and the eggs and larval stages of larger animals


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The Story of Earth: An Observational Guide by Daniel Hauptvogel & Virginia Sisson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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