The Seafloor Revealed

Glossary of Geological Terms

acoustic basement - the material which forms the base of a stratigraphic section imaged with sound waves. Although the interface between the acoustic basement and overlying material can be determined, the acoustic basement is acoustically "opaque", meaning that no structure, bedding, or other feature can be determined within this material.

acoustic facies - in seismic reflection, a portion of a stratigraphic section imaged using sound waves, which is distinct and can be distinguished from surrounding materials. In side-scan sonar imagery, a gray-scale pattern characteristic of a seafloor type.

acoustic reflectors - layers which produce characteristic "sharp" or "dark" returns on a sequence imaged using sound waves. This sharp return is caused by a difference in acoustic properties (density, composition) between adjacent geologic materials.

acoustic returns - sound waves reflected from surface or subsurface layers and recorded in the course of imaging the seafloor or a geologic sequence.

acoustic shadows - areas which have not been imaged because sound waves were blocked from reaching the area by an elevated obstruction, such as a rock.

acoustically transparent - a portion of a geologic sequence which can not be imaged using sound waves, usually due to the uniform composition of the material.

analog - a paper copy of the data, as opposed to a digital record.

armors - protects, as in having a protective covering.

bathymetric - describes the measurement of depth, usually associated with bodies of water (oceans, lakes).

bathymetric relief - the maximum change in elevation (depth) in a portion of an underwater area. Areas of high relief have steep slopes and large depth changes.

berm - the nearly horizontal portion of a beach formed by the deposition of sand by waves near the elevation of high tide.

buried valleys - valleys formed during an earlier time which are now filled with sediments.

clasts - individual sedimentary particles such as a grain of sand, pebble, or boulder that make up a sedimentary rock or deposit.

deglaciation - the process by which a glacier recedes from an area.

drumlin - a hill of till or gravel elongate parallel to the direction of glacial ice motion. The shape of the drumlin indicates the direction from which the ice advanced over the land.

Embden Formation - a sand deposit in the upper Kennebec River valley that postdates (overlies) the Presumpscot Formation and has fluvial origins.

end members - the pure constituents of a mixture. For example, sand, silt, and clay are common end members of a sediment sample.

estuaries - coastal embayments where fresh and salt water mix to produce a low-salinity water body.

estuarine conditions - refers to saline and tidal conditions which occur due to tidal ebb and flow in a region where fresh and salt water mix. Salinities and tidal ranges are typically less than those in the adjacent salt water portion of the region, but more than those in the adjacent fresh water area.

eustasy - world-wide simultaneous change in sea level.

eustatic - pertaining to global change in sea level.

facies - characteristics of a sedimentary unit that differentiates it from another. Usually representing its mode of origin or environment of deposition.

fluvial - a descriptive term pertaining to rivers; riverine.

geographic information system (GIS) - a computer-based mapping system that allows maps to be created within, or digitized into a computer, plotted and analyzed. Once entered into a GIS maps, or layers of map-related information, can be linked to a spatial database and used to create new maps.

geomorphology - a branch of geology that studies the form of the earth, the general shape of its surface, and the origin and evolution of landforms.

geophysical tracklines - the points along a vessel's course where geophysical data, such as side-scan sonar images and seismic reflection profiles were collected.

glacial-marine - pertaining to sediment originating from a glacier and deposited in a marine setting.

glacial-marine deltas - sedimentary landforms composed of glacially-derived material transported by glacial meltwater and deposited in a body of salt water.

glaciation - changing the earth's surface by the erosion and deposition of rock and sediments by a glacier.

glacigenic - formed by glaciation.

gravel aprons - a deposit composed of gravel-sized material which surrounds or partially rings a landform.

gravel-lag deposit - an accumulation of gravel-sized material which remains after finer-grained material is removed by erosion or winnowing.

ground moraine - a low-relief landform composed of a heterogeneous mixture of material deposited on the earth's surface by a glacier.

halos - ring-shaped features which surround, or partially surround a location.

heavy minerals - mineral grains which have a high specific gravity, usually greater than 2.8 times the density of water. This characteristic causes these minerals to be selectively concentrated by wave or current action. Common heavy minerals are ilmenite, magnetite, garnet, and hornblende.

highstand - the uppermost topographic position or elevation on land reached by sea level during a specific period in time.

Holocene - unit of geologic time extending from the end of deglaciation of the last ice sheet to the Present. Holocene is usually taken to represent the last 10,000 years of earth history.

hydrodynamic conditions - conditions generated by the force or pressure of moving water.

inner continental shelf - gently sloping, shallow margin of a continent extending from the shore to a location where depth increases abruptly, often around 120 m depth. Glaciated continental shelves like the Gulf of Maine have lost their cover of relatively young sediment and have exposed bedrock at the seafloor in many places. The inner continental shelf is arbitrarily defined for this report as extending from shore to the 100 m depth.

interfingering - grading or changing from one material to another in an elongate, wedge-shaped pattern.

isobath - a line of equal depth; used to map bathymetric contours.

isostatically - changes in elevation created by the weight of one material on another; gravitational balance of large portions of the earth's crust that float on a denser, underlying layer.

lithology - the physical characteristics of a rock, such as mineral composition and color.

lodgement till - heterogeneous sedimentary material deposited at the base of a glacier by moving ice. A common sediment of ground moraine.

LORAN C - LORAN is an acronym for a navigation system involving transmission of a radio signal from two or more sources. A device on a boat receives the two signals at slightly different times, and uses the "time delay" of the signals to locate a geographic position on a nautical chart which displays time delay lines as coordinates.

lowstand - the lowermost bathymetric position or depth reached by sea level during a specific period in geologic time.

marine inundation - the covering of a land area by sea water.

marine limit - the highest topographic position reached by sea water.

massive - homogeneous, without discernible layering.

Mesozoic Era - a division of geologic time extending from 245 to 65 million years ago. Preceded by the Paleozoic Era and followed by the Cenozoic Era.

moraine - a landform of glacially moved sediment (often including till) in the form of a ridge parallel to the former margin of the ice (recessional). Ground moraine refers to blanket deposits of glacial sediments of poorly defined shape.

morphology - the shape of a feature such as a landform or the seafloor.

Mya arenaria - softshell clam.

orogenic belt - a mountain range consisting of folded and faulted rocks.

outwash - sand and gravel deposited by meltwater streams beyond the melting terminus of a glacier. Outwash is layered (stratified) and generally becomes finer-grained away from the ice margin.

paleodelta - a delta formed earlier in geologic time.

palimpsest - palimpsest sediments were deposited under one set of environmental conditions (by a glacier, for example) and then strongly influenced by another set of conditions (like ocean waves) so that the sediments have lost properties imparted by the original agent of deposition.

pipette methods - a laboratory procedure of determining grain size distributions of silt and clay. This is accomplished by withdrawing a fixed volume of sediment and water at specific time intervals and depths from a specified original volume of sediment and water using a pipette. Grains of different sizes settle at different rates through a graduated cylinder.

Pleistocene - a unit of geologic time (an Epoch of the Quaternary) extending from 2 million years ago to 10,000 years ago. The most recent Ice Age.

physiographic maps - maps showing physiographic regions of an area which are identified on the basis of bathymetric relief and geology.

physiographic regions - portions of an area which are identified on the basis of bathymetric relief and composition. Similar to land terms such as coastal lowlands or interior highlands.

pockmarks - circular- to oval-shaped depressions on the seafloor caused by the release of subsurface fluids or gas from sea floor sediments.

Presumpscot Formation - a sedimentary deposit of glacial-marine mud that consists of clay- and silt-sized particles with minor amounts of sand. The sediments may include coarser gravel clasts or "dropstones" from ice-rafting.

Quaternary - the younger of the two geological periods in the Cenozoic Era. The Quaternary Period follows the Tertiary, and covers the time of many glacial advances and includes about the last two million years.

radiocarbon dates - the age of organic material determined by measuring the concentration of a naturally occurring radioactive carbon isotope, 14C. Used to establish a chronology of geologic events.

recessional moraines - linear, ridge-shaped deposits of heterogenous material deposited at the glacier front during its recession.

regression - the withdrawal of sea from land driven by a relative lowering of sea level and/or uplift of the land.

reworking - sediment is reworked when it is influenced by a process following its original deposition. Glaciers deposited sediment on the coast of Maine, and waves and tidal currents have reworked the glacial deposits into beaches and mud flats.

rock flour - finely ground rock particles or fragments resulting from glacial abrasion.

sediment ponds - relatively flat-lying depressions with a sediment filling. Sediments are often deposited in bedrock basins or channels in sufficient thickness that bedrock relief is covered.

seismic reflection - method of geophysical exploration using acoustic waves and interpretation of their reflection from submarine layers. See Figure 16A.

sessile organisms - marine animals or plants that remain in a single location and do not move about; living attached to the seafloor.

settling tube - a tall glass column filled with water into which a sediment sample is released. A balance at the base of the column measures the accumulation of grains over time to produce a grain size distribution.

shear strength - internal resistance to stress (or movement) that comes from friction and cohesion of the sediment.

shoals - shallow water that may be a hazard to navigation.

shoreface - the seaward sloping surface of the seafloor composed of sand and gravel and under the influence of wave agitation. Generally the seaward extension of the beach below the low-tide line.

side-scan sonar - a geophysical instrument that uses sound waves reflected off the seafloor to image the areal extent of different bottom types. See Figure 16B.

slumping - downslope mass movement of sediments; usually involves deformation and disruption of layers and may occur at varying rates.

stratigraphic section - a sequence of rocks or sediments. Stratigraphy is the branch of geology that studies the formation, composition, sequence and spatial correlation of stratified, or layered, rocks and sediment.

subaerially - beneath air, as opposed to beneath sea water (submarine) or fresh water (subaqueous).

subglacial meltwater - water beneath glacial ice. Water melting from the base of an ice sheet may be concentrated into tunnels under the ice and can carry sediments that are deposited in moraines and eskers (steep sinous ridges).

substrate - any layer of material below a soil or water-sediment interface.

suspect terrane - rocks that have been transported far from their place of origin by tectonic processes.

ternary diagram - a triangular graph used to plot percentages of each of three components such as sand, silt, and clay. Each apex is considered 100% of one component. See Figure 24 for examples.

texture - the size, shape, and arrangement of particles like sand, silt, and clay grains that compose a sediment.

till - non-sorted, and generally non-stratified sediment carried or deposited by a glacier. Lodgement or basal till was deposited directly beneath a glacier and is very compact and may possess clasts with a preferred orientation relating to the direction of ice motion. Ablation till is a loosely consolidated deposit and was left by melting ice.

towfish - a scientific instrument towed beneath the sea surface. Used to transmit and receive sound signals for generating side-scan sonar images.

transducer - a device that converts electrical energy to acoustic signals. May also receive sound signals and convert them to electrical signals.

transgression - a rise in the level of the sea relative to land. Can be caused by earth movement or eustasy.

unconformably - not succeeding the underlying strata in immediate order of age. Unconformable layers suggest a period of erosion or non-deposition between the two.

unconformity - in sedimentary geology, a significant gap between two deposits in the geologic record which represents a period of erosion of the lower deposit prior to deposition of the upper layer. An unconformity results in an imcomplete stratigraphic section. If the layers are at different angles to one another, the boundary is called an angular unconformity.

vibracore - a coring technique which involves pushing a vibrating pipe into sediment and removing it with a core sample intact inside the pipe.

Wisconsin - the most recent widespread glaciation known from North America.

Contents Introduction Methods Physiography Surficial Geology Geological History Summary Glossary References

Last updated on October 6, 2005