Glossary of Technical Terms Related to the Ice Age Floods
This glossary was prepared by the Ice Age Floods Institute to help people of all backgrounds understand the geologic terms used to explain phenomena related to the Ice Age Floods. For links to other glossaries available on the Web, visit the Links section of the Resources page. NOTE: Terms marked with an asterisk (*) are defined elsewhere in the glossary.
- advance outwash - sediments washed from a glacier by meltwater and deposited beyond the end of the glacier during its forward movement.
- anticline (opposite part of a syncline*) - The upwarped portion of a fold in the earth's crust; like the upward rounded linear ridges of a rumpled carpet.
- basalt - A dark igneous* volcanic rock composed of primarily two minerals: plagioclase and pyroxene. Over a period of 11 million years (17 to 6 million years B.P.) hundreds of flows of Columbia River basalt were extruded from long; linear vents in southeastern Washington; northeastern Oregon and west-central Idaho and traveled for hundreds of miles before cooling and solidifying to form the Columbia Plateau.
- bedrock - A general term for the rock (e.g.; basalt*) that underlies the soil or other unconsolidated; surficial material.
- butte - A conspicuous; isolated; generally flat-topped hill with relatively steep side slopes; often capped by a more resistant layer of rock and bordered by talus*. Often represents an erosional remnant; smaller in extent than that of a mesa*; carved from flat-lying rocks.
- cfs - cubic feet per second, common unit of measure for flow in rivers.
- Channeled Scabland - An eroded; interconnected network of streamlined loess* islands; flood channels; coulees*; cataracts; and plunge pools* scoured into basalt* by cataclysmic floods in eastern Washington State. These features are unique to this region of the Earth; however they are similar to channel networks observed on Mars.
- clast - An individual particle or fragment of a sediment or rock produced by the mechanical weathering of a larger rock mass.
- clastic dike - A feature that cuts across bedding structures and is composed of the sedimentary* material it transects. Believed to be the result of fracturing and sediment movement due to earthquake shaking during or soon after cataclysmic flooding.
- clay - Extremely small sedimentary* particles that are less than 0.004 mm in diameter.
- coarse-grained - Pertains to sedimentary* material composed of relatively large particles of sand* and/or gravel*.
- columnar basalt - Thick lava flows commonly appear to be comprised of vertical honeycomb-like columns of basalt. Linear joints form in the rock as it cools. They define the columns and generally form perpendicular to the cooler bounding surfaces above and below. In cross section the resulting joints are often six-sided (an optimum packing geometry) but may be anywhere from 3-sided to nearly round. Because of the through-going joining; columns are relatively easily broken out of a rock face by hydraulic plucking and ice-wedging.
- continental glacier - a thick ice sheet covering large parts of a continent.
- coulee - A long; dry; steep-walled; trench-like gorge or valley representing an abandoned river channel. In south central Washington; the term coulee is mostly used for an abandoned ice-age flood channel.
- diorite - a plutonic rock of intermediate composition between granite and gabbro, chemically equivalent to andesite.
- DNR - Washington State Department of Natural Resources.
- drumlin - an elongated hill, comprised of previously-deposited glacial drift (or solid rock), shaped by subsequent glaciation.
- elev - surface elevation (feet) above present-day sea level for land, water, and ice.
- end moraine - a ridge of till marking the standstill (end) position of a present or past glacier.
- eolian - Pertaining to the wind. Includes deposits of loess* and dune sand*.
- erratic - An erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. implying it has been transported and deposited at some distance from the outcrop from which it was derived and generally associated with transport by glacial ice or icebergs..
- esker - a sinuous ridge of roughly stratified sand and/or gravel deposited by a stream flowing beneath the ice of a stagnant glacier.
- eustacy - a uniform worldwide change in sea level caused by fluctuations in the amount of water taken up by continental and polar icecaps, or by a change in the capacity of ocean basins.
- expansion bar - Expansion bars consist of broad, laterally spread flood sediment deposits in wider portions of a channel immediately downstream from a more confined channel section. These deposits are thought to result from the slackening of debris-charged floodwaters as they spill out of the more confined channel into the broader channel sections where the rate of flow slackens.
- flood bar - An accumulation of sediment; most often composed of sand* and/or gravel*; that occurs along flood routes where the currents move slower for various reasons. Different types of flood bars include eddy bars; expansion bars*; shoulder bars; and pendant bars*.
- flood basalt - a prodigious outpouring of very fluid basaltic lava, typically from large fissure eruptions, that rapidly spreads out across the landscape, forming a broad and often thick sheet of lava that cools and hardens into a broad, laterally continuous layer of basalt. A given flood basalt is often one in a stacked series of flood basalts forming a continental flood basalt over a relatively short geologic time period.
- fluting - landform created by movement of a glacier across previously-deposited sediments, resulting in long, streamlined ridges on the ground aligned parallel to the direction of ice flow.
- fore-set bedding - Primary sedimentary* structure in flood gravels where a pronounced dip occurs in bedding planes in the direction of sediment transport.
- foreset bed - the inclined part of a river delta at the end of the stream channel which delivers sediment to a still body of water.
- giant current ripples (GCRs) - Giant current ripples are active channel topographic forms up to 20 m high, which develop within central flow areas of the main outflow valleys created by glacial lake outburst floods. Giant current ripple marks are extremely large analogs of small current ripples formed in sandy stream sediments. Giant current ripples are important diagnostic depositional features in plains and scablands associated with massive floods.
- glacial advance and retreat - the terminus, or snout, of a glacier periodically retreats or advances, depending on the amount of snow accumulation vs. evaporation or melt that occurs. Even as the terminus advances or retreats, the ice comprising the glacier still internally deforms and moves downslope as if on a conveyor belt.
- glacial drift - material transported directly by a glacier.
- glacial groove - a furrow cut into bedrock by abrading action of rock fragments embedded in a glacier.
- glacial lake - a temporary lake formed behind, or in front of, an ice dam. A proglacial lake forms in front of an advancing glacier, a glacial lake remains when the glacier stops moving, and a recessional glacial lake follows a retreating glacier.
- Glacial Lake Bretz - the recessional glacial lake which remained after Glacial Lake Russell drained into the Strait of Juan de Fuca, lowering the water level remaining in the Puget Lowland. When Glacial Lake Bretz drained, water level in the Puget Lowland matched sea level.
- Glacial Lake Missoula - The source for most or all of the floodwater that created the Channeled Scabland* of eastern Washington. The lake formed behind an ice dam in the Idaho Panhandle that periodically failed; sending torrents of water downstream. At its maximum; Lake Missoula contained 600 cu. mi. of water; was 2;000 ft deep; 200 miles long; and covered an area of 3;000 sq. mi. It took up to 125 years to fill but only 2 to 3 days to completely empty.
- Glacial Lake Russell - the proglacial lake which formed south of the advancing and retreating Puget Lobe when meltwater discharge to the Strait of Juan de Fuca was blocked by the Juan de Fuca Lobe.
- glacial maximum - the time in glacial history during which an ice sheet reaches its greatest extension; for the Puget lobe during the Vashon glaciation, this time was about 16,900 years ago.
- glacial striation - a scratch or very small groove cut into bedrock by abrading action of rock fragments embedded in a glacier.
- glacial trough - a U-shaped valley, often with steep, straight sides and a flat bottom, formed by the process of glaciation. In the Puget Sound region, the larger troughs now lie mostly below sea level.
- granitic - Pertaining to granite; a general term for any light-colored igneous* rock that formed deep underground within a cooling body of liquid magma.
- granodiorite - A type of granitic* rock consisting of mostly crystalline quartz and plagioclase feldspar.
- gravel - Large sedimentary* particles that are greater than 2 mm in diameter. Gravel clasts* include; in increasing size; granules; pebbles; cobbles; and boulders.
- hanging valley - A tributary valley whose floor is notably higher than the valley it joins. Characteristic of flood coulees*; where flat valley floors suddenly drop off abruptly at one or both ends where they join adjacent coulees.
- Holocene Epoch - The period of geologic time since the last Ice Age (10;000 years ago to the present).
- hydraulic constriction - Where a large volume of water is confined to a narrow opening. If more water enters the opening than can drain through; then the constriction will cause water to back up; creating a type of hydraulic dam.
- Ice Age - The last glacial period is popularly known as the Ice Age, and was the most recent glacial period within the Quaternary glaciation occurring during the last 100,000 years of the Pleistocene, from approximately 110,000 to 12,000 years ago. Scientists consider this "ice age" to be merely the latest glaciation event in a much larger ice age, one that dates back over two million years and has seen multiple glaciations. During this period, there were several changes between glacier advance and retreat. The Last Glacial Maximum, the maximum extent of glaciation within the last glacial period, was approximately 22,000 years ago
- ice elevation - elevation of ice surface (feet) above present-day sea level. Ice thickness equals ice elevation minus elevation of the ground on which the ice lay.
- ice-marginal - refers to the area along the side or front of a glacier.
- igneous - Rock that solidified from molten or partly molten material (i.e.; magma). One of the three principal rock types; along with sedimentary* and metamorphic*.
- isopach - a contour line of equal thickness (of ice, rock, or other geologic structure) over an area. An isopach map illustrates thickness variations within a unit such as a glacier.
- isostatic rebound - also called post-glacial rebound or crustal rebound, is the rise of land masses that were depressed by the weight of ice sheets during the previous glacial period, through a process known as isostatic depression.
- jökulhlaup - a sudden release of meltwater from a glacier or glacier-dammed lake, a glacial outburst flood.
- ka, Ma - abbreviations for thousand years before present, and million years before present.
- kame - a mound or short irregular ridge composed of stratified sand or gravel.
- kettle - a closed depression in a deposit of glacial drift formed where a block of ice was buried and then melted.
- kettle lake - a kettle depression, left behind after partially-buried ice blocks melt, that has subsequently been filled with water.
- Kolk - An underwater vortex created when a rapidly rushing current of water passes an underwater obstacle in boundary areas of high shear. High velocity gradients produce a violently rotating column of water; essentially a high energy whirlpool; that can pluck multi-ton rocks from the underlying bed of the current. A Kolk is usually relatively stationary due to the factors that create it; and is the mechanism that created Kolk ponds during the Floods.
- Lake Bonneville - An Ice-Age lake that formed in central Utah from melting mountain glaciers. The Great Salt Lake today is a much smaller remnant of Lake Bonneville. The lake drained catastrophically only once; toward the end of the Ice Age about 15;000 years ago when the lake overtopped a drainage divide and partially drained northward into the Snake River.
- Lake Lewis - A temporary lake that formed behind the hydraulic constriction* at Wallula Gap*. Within 5 days or less the lake grew to an elevation of 1;250 ft above sea level before completely draining through the gap over a period of several days.
- Large Igneous Province (LIP) - large volumes of mainly mafic magmas occurring as continental flood basalts, volcanic rifted margins, oceanic plateaus, and ocean basin flood basalts. The formation of LIPs is variously attributed to generation and emplacement by mantle plumes or to processes associated with divergent plate tectonics.
- lateral moraine - a moraine marking the left- or right-side position of a glacier, as seen facing in ice flow direction.
- lee - the side of a hill in the direction of movement of a glacier.
- lidar - acronym for Light Detection and Ranging, a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth. It is commonly used to create mapping which shows surface relief.
- lithology - The physical character of a rock; including its color; mineralogic composition; and grain size.
- loess - Windblown silt* and fine sand* that collects on the lee sides of ridges at higher elevations within the Pasco Basin.
- maar - A broad, low-relief volcanic crater caused by an eruption when groundwater comes into contact with hot lava or magma. Maars are shallow, flat-floored craters that scientists interpret as having formed above a volcanic pipe as a result of a violent expansion of magmatic gas or steam. Most maars have low rims composed of a mixture of loose fragments of volcanic rocks and rocks torn from the walls of the diatreme.
- magnetic polarity shifts - A change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was the opposite. The time spans are randomly distributed with most being between 0.1 and 1 million years with an average of 450,000 years. Most reversals are estimated to take between 1,000 and 10,000 years.
- meltwater - water released by melting ice and snow.
- mesa - An isolated; nearly level land mass standing distinctly above the surrounding country; bounded by abrupt steep-sided slopes on all sides and capped by layers of more-resistant rock.
- metamorphic - Any rock derived from pre-existing rocks by mineralogical; chemical; and/or structural changes. One of the three principal rock types; along with sedimentary* and igneous*.
- Miocene Epoch - The period of geologic time between 5 and 24 million years before the present; when Columbia River basalt* was extruded into the Pasco Basin.
- moraine - a glacially formed mass of unconsolidated glacial debris, rocks and sediment, carried down and deposited by a glacier, typically as ridges at its edges or extremity.
- nunatak - an isolated knob or peak of bedrock that projects prominently above the surface of a glacier.
- ophiolite - a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level by thrusting (obduction) onto the edge of a continental plate.
- outwash - sediments (clay, silt, sand, gravel, or boulders) washed from a glacier by meltwater and deposited beyond the end of the glacier.
- paleomagnetism - The fossil magnetism in rocks, used to determine the past configurations of the continents and to investigate the past shape and magnitude of the earth's magnetic field. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates.
- paleosol - Very old; buried soil.
- patterned ground - Well-defined; more or less symmetrical forms; such as circles; polygons; nets; steps; and stripes; that are characteristic of; but not necessarily confined to; surficial material subject to intense frost action; especially in polar; subpolar; and arctic regions. Patterned ground in the Pasco Basin; however; appears to be related to seismicity* that occurred during or soon after cataclysmic flooding.
- pedogenic - Relating to the processes that produce soil.
- pendant bar - A type of flood bar* that forms immediately downstream of an obstruction in the flow of the flood current.
- periglacial - relating to or denoting an area adjacent to a glacier subject to repeated freezing and thawing.
- Pleistocene Epoch - The period of geologic time between 10;000 and 2.5 million years before the present. The Pleistocene essentially spans the same period of time known as the Ice Age.
- Pliocene Epoch - An epoch of the Tertiary period; after the Miocene and before the Pleistocene.
- plucking - also referred to as quarrying; a glacial phenomenon responsible for erosion and transportation of individual pieces of bedrock, through the action of freezing water which pries loose pieces of bedrock and attaches them to the moving ice.
- plunge pool - A deep pool or basin formed at the foot of a waterfall.
- pluvial lake - A lake formed in the Pleistocene epoch* during a time of glacial advance; and now either extinct or existing as a remnant.
- point bar - An arcuate (curved or bow-shaped) ridge of sand* and gravel* developed on the inside of a growing meander (loop) in the course of a stream.
- pro-glacial - in front of, at, or immediately beyond the margin of a glacier or ice sheet, such as a pro-glacial lake lying in front of a glacier.
- radiometric - Refers to methods of age determination based on the nuclear decay of radioactive isotopes.
- recessional outwash - sediments washed from a glacier by meltwater and deposited beyond the end of the glacier during its final retreat (melting or wasting).
- relict (glacial relict) - a structure or rock that did not undergo change following retreat (melting) of a glacier.
- retreat (glacial retreat) - wasting, melting, ablation of glacial ice.
- reverse grading - Refers to sedimentary* beds that show an increase in particle size upward within the bed; as opposed to normal grading which shows a decrease upward. Most flood rhythmites* display normal grading.
- reverse or thrust fault - A fault; usually with a dip of >45 degrees; where the hanging wall has moved up relative to the footwall of the fault.
- rhythmite - A graded sedimentary* layer; several inches to several feet thick; deposited under slackwater* conditions; especially in backflooded valleys during cataclysmic floods. Some believe that each rhythmite represents a separate cataclysmic flood from glacial Lake Missoula*.
- Ringold formation - Sediments stratigraphically overlying Columbia River basalt* and underlying cataclysmic flood deposits in southeastern Washington. Mostly derived from ancient river and lake deposits that accumulated within the ancestral Columbia River basin between about 8.0 million and 3.4 million years ago.
- rip-up clast - Sedimentary* material that has been eroded and transported only a short distance in a semi-consolidated (e.g.; frozen) state.
- sand - Sedimentary* particles that are between 0.06 to 2.0 mm in diameter.
- Sangamon Interglacial - The time period between glacial stages; roughly 130;000-80;000 years ago; when climatic conditions were similar to those of today.
- scabland(s) -
Types of terraine or regione found, for example, in the NW US, characterized by tracts of rough, barren, rocky land with bare igneous rock surfaces, little or no soil cover and little vegetation, that are scarred, traversed or isolated by deep postglacial dry stream channels formed by glaciers or glacial floods — (geology, usually used in plural form)
- sedimentary - Composed of sediment. One of the three principal rock types; along with igneous* and metamorphic*.
- seismicity - Earth vibrations and shaking due to earthquake activity; as well as those artificially induced.
- silt - Small; sedimentary* particles between 0.06 to 0.004 mm in diameter.
- slackwater - Refers to areas with slower moving flood waters associated with cataclysmic flooding (i.e.; backflooded valleys and valley margins) where fine-grained sediment (mostly sand* and silt*) was deposited.
- soft-sediment deformation - Deformation that occurs during or soon after sediment deposition while sediment is still partially or fully saturated with water. Examples of soft-sediment deformation include flame structures; load structures; and clastic dikes*.
- soil horizon - A distinct interface (surface or thin layer) in a stratigraphic sequence.
- spillway - a natural drainage channel cut by water from a melting glacier or ice field.
- stoss - the side of a hill facing the onward movement of a glacier.
- subglacial - Formed or accumulated in or by the bottom parts of a glacier or ice sheet.
- syncline (opposite part of an anticline*)
- talus - Broken rock accumulated at the base or against the lower part of a steep slope or cliff.
- tectonic - of or relating to the structure of the earth's crust and the large-scale processes that take place within it.
- tephra - Airfall deposit from a volcanic eruption. Usually consists of distinctive; light-colored; well-sorted; gritty particles of ash.
- till (glacial till) - unstratified drift, deposited directly by a glacier without reworking by meltwater, consisting of clay, silt, sand, gravel, and boulders ranging widely in size and shape.
- Vashon Glaciation or Stade - the period from 19,000 to 16,000 years ago when the last major glacial advance into the Puget Sound Lowland occurred.
- venturi effect - The principle that fluid moving through a smaller area will move at a higher velocity than the same amount of water moving through a larger area. As an example; floodwater moving through a narrow opening such as Wallula Gap* was moving much faster; with significantly more erosive power; than the water above or below the gap
- Wallula Gap - The narrow constriction; only a few miles wide; through which all floodwaters from glacial Lake Missoula* passed on their way to the Pacific Ocean. During the largest floods; the water within Wallula Gap was over 1;200 ft deep.
- Wisconsinan - Pertaining to the classical fourth and last glacial stage of the Pleistocene Epoch* in North America; following the Sangamon interglacial* and preceding the Holocene Epoch*. The late Pleistocene Wisconsinan glacial stage occurred between about 80;000 and 15;000 years before the present.