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GLOSSARY

Most of the definitions below are based in part on the Glossary of Geology, second edition, published by the American Geological Institute. Additional definitions are from the Dictionary of Geological Terms, Bates and Jackson, 1984.

A

alluvium   Sediment eroded from adjacent areas and deposited by running water in and along rivers and streams.

aquifer   A saturated sediment or rock that is sufficiently porous and permeable to be useful as a source of water and that provides a generally  sustainable yield of suitable quantities of ground water.

areal   (as in areal distribution) Refers to the geographic area over which a rock or sediment unit occurs, either buried or exposed at the surface.

B

beach ridge   A low, essentially continuous mound of beach and dune material heaped up by the ction of waves and currents on the backshore of a beach beyond the present limit of storm waves or of ordinary tides, and occurring singly or as one of  a series of approximately parallel deposits.  They represent successive positions of an advancing shoreline.

bed   A layer of sediment or sedimentary rock; see strata.

bedding plane   The division plane that separates each successive layer of sedimentary or stratified rock.

bedrock   The solid rock that underlies unlithified sediment of glacial and other origins.

brachiopod   Any marine invertebrate belonging to the phylum Brachiopoda, characterized by two bilaterally symmetrical valves that are commonly attached to a substratum but may also be free.

C

calcareous   Containing calcium carbonate.

chert   A hard, dense microcrystalline sedimentary rock, consisting chiefly of interlocking microscopic crystals of quartz; occurs principally as nodules in limestone and dolostone.

correlate   To show a definite correspondence in character and stratigraphic position between geologic formations in two or more separated areas.

cross section   A diagram or drawing that shows the sequence of rocks and sediment layers as they occur in a vertical plane; commonly drawn from the ground surface down to some selected depth, such as the bedrock surface.

cuesta   A ridge with a gentle slope on one side and a steep slope on the other.

D

diamicton  A general term for unsorted, unstratified rock debris composed of a wide range of particle sizes; use of this term carries no suggestion about how such debris was formed or deposited.

dip slope   A slope of the land surface that conforms approximately with the dip of the underlying rocks; specifically, the long, gently inclined face of a cuesta.

doline    A closed depression formed by one of several processes.  Collapse doline forms when a cavity is hollowed out below the surface by dissolution of carbonate rocks and subsequently collapses.  Subsidence doline is formed when thin cover of carbonate rocks collapse into an underlying dissolved cavity.  See sinkhole.

dolomite (dolostone)   A sedimentary rock (calcium magnesium carbonate) similar to limestone (calcium carbonate) and especially common in the bedrock of northeastern Illinois.

dolomitic limestone   A limestone in which the mineral dolomite is conspicuous, but calcite is more abundant

drumlin  A low, smoothly rounded, elongate hill of compact glacial till built under the margin of the ice and shaped by its flow;  its longer axis is parallel to the direction of ice movement.  It has a blunt nose which points in the direction from which the ice approached, and a gentler slope tapering in the other direction.

E

end moraine   A ridge formed by the accumulation of glacial drift at the edge of a glacier. In Illinois, most end moraines are composed predominantly of till.

erratics   Boulders and other rock fragments transported by glacial ice from their place of origin to an area where the bedrock is different.

escarpment   A long, more or less continuous cliff or relatively steep slope facing in one direction.

esker   A long, narrow, sinuous ridge of sand and gravel deposited by a melt water stream flowing upon, within, or beneath a glacier that is melting away.

F

formation   The basic unit of lithostratigraphic classification. A formation must be identifiable on the basis of easily recognized physical properties and widespread enough to be mapped at a regional scale. A formation can be divided into smaller units (called members) where these, too, are recognizable and mappable. Formations can also be combined into groups when useful and appropriate.

fossil   Any remains, trace, or imprint of a plant or animal that has been preserved in the earth’s crust since some past geologic or prehistoric time.

G

geologic map   A diagram or drawing on a horizontal plane of part of the earth's surface showing by means of lines, colors, symbols, and orientation thedistribution of selected features such as particular surface or subsurface units of rocks or sediments.

glacial   Pertaining to the activities of glaciers, or to the features or materials produced thereby.

glacial scour   The eroding action of a glacier, including the removal of surficial material and the abrasion and polishing of the bedrock surface by rock fragments dragged along by the ice.

glaciation   The formation, movement, and recession of glaciers or ice sheets.

glacial striations   A series of long, generally straight and parallel scratches or furrows on a bedrock surface which were caused by the dragging and scraping of rock fragments that were frozen into the base of an overriding glacier; striations are usually oriented in the direction of ice movement. Glacial striations are also formed on erratics (rock fragments) transported by the ice.  

groundmass   A term sometimes used for the matrix of a sedimentary rock.

ground moraine   A gently rolling ground surface underlain by till deposited beneath a glacier. It is usually bordered by end moraines.

groundwater   See aquifer.

group   The major lithostratigraphic unit next higher in rank than a formation; a group consists of two or more associated and adjoining formations having significant lithologic features in common.  

H

I

igneous   One of the three basic categories into which rocks can be classified, of which the other two are sedimentary and metamorphic. Igneous rocks are formed by the cooling of molten rock, called magma.

isostasy   The condition of equilibrium, comparable to floating, of the units of the lithosphere (crust) above the athenoshpere. 

isostatic rebound   The return to equilibrium after some change in the weight of the crust; i.e. ice sheets.

J

joint   A surface of fracture or parting in a rock, without displacement .

K

karst  topography   A type of topography that is formed over limestone, dolomite or gypsum by dissolution, and that is characterized by sinkholes, caves, and underground  drainage.

L

laminated   very thinly layered

leaching   The dissolving and removal of soluble minerals by water from rain and snow that infiltrates downward through a soil.

limestone   A sedimentary rock consisting chiefly of the mineral calcite (calcium carbonate).

lithology  The description of rocks, esp. in hand specimen and in outcrop on the basis of such characteristics as color, mineralogic composition, and grain size.

lithostratigraphic unit   A stratigraphic unit defined and identified by lithologic features without regard to origin, fossil content, or time boundaries.

loam  A rich, permeable soil composed of a mixture of clay, silt, sand and organic matter.

M

matrix   The finer-grained material enclosing the larger grains in a sedimentary rock or sediment.

metamorphic   One of the three basic categories into which rocks can be classified, of which the other two are sedimentary and igneous. Metamorphic rocks are those changed in composition, mineral content, texture, or structure by the application of heat or pressure; they originally may have been sedimentary, igneous, or other metamorphic rocks.

moraine   A mound or ridge of unstratified glacial drift, chiefly till, deposited by direct action of glacier ice.

N

O

orogenic  Associated with a mountain building or tectonic event.

organic   Biologically derived materials containing carbon as an essential component, usually bonded with hydrogen. Organic geologic materials include wood, shells, peat, and bone.

outcrop   Part of a glacial deposit or bedrock that is exposed and visible at the earth's surface.

outwash   Stratified sand and gravel that was washed out from a glacier by melt water streams and deposited in front of, or beyond the margin of, an active glacier .

overburden   Sediment or rock that overlies an economically useful deposit and which must be removed prior to mining that deposit.

P

paleo-    A combining form meaning old or ancient.

peninsula   A body of water nearly surrounded by water, and connected with a larger body by a neck or isthmus; any tract of land jutting out into the water.

permeability   The capacity of a porous rock, sediment, or soil for transmitting a fluid; a measure of the relative ease of fluid flow under unequal pressure.

proglacial   Immediately in front of, or just beyond the outer edge of, a glacier; proglacial refers to lakes, streams, deposits, and other features produced by or derived from glacial ice.

Q

Quaternary   The geologic period beginning two to three million years ago and extending to the present.

R

regression  retreat of the sea from land areas; any change that converts offshore deep-water conditions to near-shore shallow water conditions.  The movement of the shoreline basinward, away from the land.

relief   A term used loosely for the general unevenness of the earth's surface; more precisely, it refers to the vertical difference in elevation between hilltops and valleys of a given region. A region showing little variation in elevation has low relief.

S

sandstone   A clastic sedimentary rock composed of sand sized grains set in a matrix of silt or clay, and firmly united by a cementing agent (silica, iron oxide, or calcium carbonate)

sediment   Solid material that has settled down from a state of suspension in a liquid; solid fragmental material transported and deposited by wind, water, or ice, chemically precipitated from solution, or secreted by organisms, and that forms in layers in loose unconsolidated form.

sedimentary   One of the three basic categories into which rocks can be classified, of which the other two are igneous and metamorphic. Sedimentary rocks are formed by the compaction and cementation of sediment or by the precipitation of dissolved minerals from salt or fresh water.

shale   A fine-grained detrital sedimentary rock, formed by the compaction of clay, silt, or mud.

silicification   The introduction of, or replacement by, silica, esp. in the form of fine-grained quartz, chalcedony, or opal, which may fill pores and replace existing minerals.

Silurian   A period of the Paleozoic Era, thought to have covered the span of time between 440 and 400 million years ago.  This period of geologic history follows the Ordovician period and precedes the Devonian period.

sinkhole   A circular depression in a karst area.  Its drainage is subterranean, its size measured in meters or tens of meters, and it is commonly funnel-shaped.

strata   Single and distinct layers, or beds, of sediment or sedimentary rock that are easily distinguishable from layers above and below them.

stratigraphic, stratigraphy   Refers to the systematic definition and description of major and minor natural divisions of rocks and their arrangement according to their composition, distribution, correlation, and mutual relationships.

stratigraphic unit  A stratum or body of strata recognized as a unit for description, mapping, or correlation.

T

tabular   Said of a  feature having two dimensions that are much larger or longer than the third.

tectonic  Pertaining to the forces involved in, or the resulting structures of, tectonics

tectonics  A branch of geology dealing with the broad architecture of the outer part of the earth, that is, the major structural or deformational features and their relations, origin, and historical evolution.

temperate glacier   A glacier characteristic of the temperate zone; its ice is near the melting point.

till   Unsorted, unstratified rock debris composed of a wide range of particle sizes that was deposited directly by and underneath a glacier.

topography   The general shape of a surface, such as the land surface, including its relief and the position of its natural and manmade features.

transgression   The spread of sea over land areas; also any change that brings offshore, deep-water environments to area that were once near shore, shallow- water environments.  The movement of the shoreline towards the land.

U

unconformity   A break or gap in the geologic record, such as an interruption in the normal sequence of deposition of sedimentary rocks, or a break between eroded metamorphic rocks and younger sedimentary rocks.

V

W

wave-cut platform   A gently sloping surface produced by wave erosion, extending far into the sea or lake form the base of the wave-cut cliff.

weathering   The process of physical disintegration and chemical decomposition whereby earth and rock materials are changed in color, texture, composition, firmness, or form upon exposure to the atmosphere.

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