Ice Age Floods Institute Masthead

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.

anticline
An upwarped portion of a fold in the earth's crust.

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.

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*.

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.

eolian
Pertaining to the wind. Includes deposits of loess* and dune sand*.

erratic
A rock fragment carried by floating ice, deposited at some distance from the outcrop from which it was derived and generally composed of a different type of rock than the local bedrock*.

expansion bar
A type of flood bar* that forms where flood channels suddenly widen or expand in size. Channel expansion causes the current to decrease, which in turn causes sediment to settle out of suspension, forming a bar. Priest Rapids Bar is such a formation.  

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*.

fore-set bedding
Primary sedimentary* structure in flood gravels where a pronounced dip occurs in bedding planes in the direction of sediment transport.

giant current ripples (GCRs)
Extremely large waveforms created by the transport and deposition of coarse-grained* flood material during Ice Age floods. They develop at right angles to the direction of flow, much the same as the smaller ripples we see along modern beaches and river bottoms. GCR's, composed of coarse-grained sand* and gravel*, have wavelengths of 100-400 ft and amplitudes of 3-25 ft (Baker 1978). The dimensions of the ripples give scientists a method for estimating the depth and speed of the flowing water that created them.

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.

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 period of geologic time between 10,000 and 2.5 million years before the present, marked by increased glacial coverage of the earth's surface. The Pleistocene Epoch essentially spans the same period of time as the Ice Age.

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*.

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.

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.

magnetic polarity shifts
Reversals in the Earth's magnetic field that have occurred periodically through geologic time. The last such reversal occurred 780,000 years ago.

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.

paleomagnetic
Pertains to the natural remnant magnetization of rock and sediment, which can be measured to determine the intensity and polarity of the Earth's magnetic field in the geologic past

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.

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.

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.

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.

radiometric
Refers to methods of age determination based on the nuclear decay of radioactive isotopes.

reverse fault
A fault, usually with a dip of >45 degrees, where the hanging wall has moved up relative to the footwall of the fault.

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.

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.

Sangamon Interglacial
The time period between glacial stages, roughly 130,000-80,000 years ago, when climatic conditions were similar to those of today.

sand
Sedimentary* particles that are between 0.06 to 2.0 mm in diameter.

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.

subglacial
Formed or accumulated in or by the bottom parts of a glacier or ice sheet.

talus
Broken rock accumulated at the base or against the lower part of a steep slope or cliff.

tectonic
Pertains to forces within the earth's crust that give rise to earthquakes, folds, faults, and joints observed at or near the surface.

tephra
Airfall deposit from a volcanic eruption. Usually consists of distinctive, light-colored, well-sorted, gritty particles of ash.

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.