Ice Age Floods
Study of Alternatives
Appendix B—Glossary of Terms


Back-arc Spreading—The region of crustal spreading between a volcanic archipelago and the continental interior.

Bars—Eddy or Shoulder Bars—Formed downstream from promontories or recesses.

Pendant Bars—Extended downstream behind the protection of a rock island or other obstruction.

Expansion Bars—Formed where the water spreads out after passing through a narrow point and deposits material.

Basalt—A dark-colored fine-grained extruded volcanic rock, rich in iron and magnesium, that is chiefly composed of plagioclase and pyroxene.

Basin—A large circular or elliptical depressed area in a landscape.

Braided River—A stream system so overloaded with sediment that there are many dividing and rejoining channels.

Colonnade—The lower portion of a lava flow with well-formed parallel shrinkage columns, usually vertical.

Columnar Jointing—Long joints, usually vertical, in volcanic rock that splits into columns as the rock cools and contracts.

Continental Glacier—A thick ice sheet covering a large part of a continent.

Coulee—A dry trench-like intermittent streambed or wash.

Delta—A low, flat tract of land at the mouth of a river, composed of stream-deposited sediment, and triangular in shape from an aerial view.

Entablature—The upper portion of a lava flow that displays irregular thin shrinkage columns.

Erratic—A large rock that is not of local origin and has been moved to its location by glacial ice.

Fault—A break in the earth’s crust along which movement has taken place. Frequently occurs as a zone of numerous smaller faults.

Fault Trace—Intersections of faults on the ground surface (horizon); also called fault line.

Fauna—The entire animal population living in a given area, environment, or time span.

Flood Basalt—A plateau basalt. Successive flows of high temperature fluid basalt from fissure eruptions merge to form a continuous flat plateau.

Flora—The entire plant population living in a given area, environment or time.

Floodplain—The flat area where a stream or river may overflow.

Glacier—A mass of ice and snow that moves downhill because of gravity.

Graded Bedding—Rapid sedimentation leaving a distinctive texture of coarse grains followed by fine grains in a single layer. See Rhythmites.

Hanging Valleys—Smaller glacial valleys feeding into a deeper main glacier trough.

Jökulhlaup—An ice dam failure or glacial outburst

Kolk Lake—A hole eroded by rushing water at the base of a broken dike; a deep pool, eddy or scour area which has filled with water.

Lithosphere—The outer 60 miles of the earth between the crust and mantle where the rocks are harder and more brittle.

Lode—Deposits in which ores have been emplaced into rock formations where they occur in veins.

Loess—Non-stratified silt, clay, and dust, originating as glacial sediment, but re-deposited by wind. Wind-blown silt.

Magma—A silica-rich molten fluid that appears on the surface as lava and that cools underground as a pluton.

Magma Chamber —A magma reservoir in the shallow part of the lithosphere.

Plunge Pool—A spot at the base of a waterfall where the descending force of the water strikes the bottom and carves out a basin. A deserted plunge pool is one whose waterfall has migrated upstream.

Pothole—A bowl-shaped depression in a rock surface that has been carved by the whirling action of stones in a stream bed.

Rhythmites—Layers consisting of gravel or sand at the base, followed by silt on top, representing separate flooding events.

Rift—A distinct, roughly linear trough or depression on the earth’s crust that reflects a pulling apart of the crust due to tension.

Ripple Marks—Deposits left by currents flowing over the bottom that shape the sediments into smooth, parallel, ridge rows resulting in washboard-like ripples that are regularly patterned.

Sand—Rock and mineral fragments ranging in size between 1/16 and 2 millimeters in diameter.

Scabland—An irregular land surface of basalt that has been scoured of its soil cover by floods.

Spillways—Low divides that have been overtopped by floodwaters.

Strand—Beach and very shallow coastal area dominated by shoreline processes, particularly wave processes.

Subduction Trench—A distinct linear groove on the earth’s crust marking the boundary between two colliding tectonic plates in which one plate slips beneath another.

Submarine Canyon—A steep V-shaped trench along or across a continental shelf or deep ocean floor.

Till—Loose sediment deposited by glaciers.

Watershed—The total area drained by a stream system.

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