Eastern Ridges and Lowlands
Lake Superior Lowland
The Physical Geography of Wisconsin, by Lawrence Martin
Readers will find that Professor Martin did an excellent job of explaining unfamiliar terms as they occur throughout the book, and that detailed technical definitions are not necessary to follow the flow of his narrative.
Nevertheless, part of the pleasure of a work such as this is the discovery of a new vocabulary that helps us understand and talk about natural phenomena. The terms describing the topographical effects of glacial activity, in particular, are essential to anyone who wishes to describe or discuss the Wisconsin landscape.
Here is a list of some terms used in this book, with explanations of each. Explanations drawn from the book are shown in quotes with a page number citation following in parentheses.
We have also found it instructive to reproduce Professor Martin's Table Showing the Geological Column, which explains terms referring to geologic eras and epochs.
- An arch of stratified rock in which the layers bend downward in opposite directions from the crest.
- Alluvium, Alluvial (adj)
- Clay, silt, sand, gravel and similar materials deposited by rivers and streams.
- An earthy material that is plastic when moist but hard when fired, composed mainly of extremely fine plate-like mineral particles.
- Basically, a hill or ridge with a steep face on one side and a gentle slope on the other. "A cuesta is an upland belt with a short, steep descent, or escarpment, on one side and a long, gentle slope on the other. The gentle slope usually corresponds to the inclination or dip of slightly- inclined sedimentary rocks. One resistant layer, as of limestone, may determine the whole dip slope." (p 44)
- Dendritic (adj)
- Describes a branching, tree-like structure. Often used to refer to the pattern of streams which have formed their courses in level, homogeneous material, instead of following cracks and fissures in more resistant rocks.
- Materials deposited or laid down by natural processes of accumulation.
- Loose material such as rock fragments and organic particles, that result directly from disintegration of the parent material. Synonymous with debris.
- Rock debris deposited by natural agents, specifically a deposit of clay, sand, gravel and boulders transported by a glacier or by running water from a glacier. "Older drift" is the term used to describe drift deposited by glaciers prior to the Wisconsonian glaciation.
- An oval or elongated hill of glacial drift. The long axis of the hill shows the direction of glacial movement. The blunt end of the drumlin points "upstream" (toward the origination of glacial movement), and the more tapered end of the drumlin points "downstream." Drumlins probably migrated across the landscape with, but more slowly than, the glaciers which created them.
- Boulders transported from their origin by a glacier, so that they are now found far from their parent rock and are now isolated amid dissimilar materials. Erratics found in Wisconsin have been transported from as far away as Minnesota, Michigan and Canada.
- A long cliff or steep slope separating two comparatively level or more gently slping surface, resulting from erosion or faulting.
- A long, narrow, often curving ridge or mound of sand, gravel and boulders deposited by a stream flowing on, within or beneath a stagnant glacier.
- Exhume (v)
- To disinter, excavate, reveal or restore. Used in the book to refer to geologic structures buried by glacial debris and subsequently revealed again by weathering and erosion.
- Glacial Period
- "The Glacial Period may have lasted more than a million years. It ended in the geological yesterday, perhaps only 35,000 to 50,000 years ago." (p 13)
- A layered metamorphic rock. Pronounced "nice."
- "Literally a grave, but used as a technical term by geologists to indicate a depression between parallel fault lines." (p 430)
- Igneous (adj)
- Rock formed by cooling and solidification of molten magma.
- A short ridge, mound or (sometimes steeply conical) hill of stratified glacial drift deposited by a crack or hole in a stagnant glacier.
- A steep-sided hollow without surface drainage, usually created by the melting of a block of ice buried in glacial drift.
- Lacustrine (adj)
- Formed by lakes, or having to do with lakes.
- Soil consisting of a friable (easily crumbled or pulverized) mixture of varying proportions of clay, silt and sand. Loam is an ideal soil type for many agricultural purposes. Wisconsin's "state soil" is Antigo silt loam, the basis for the success of much of our agricultural economy.
- An unstratified, usually buff to yellowish brown, loamy deposit created by fine mineral particles carried by the wind. Pronounced "less."
- Metamorphic (adj)
- Rock produced by changes in the original material affected by pressure, heat and water that result in a more compact, crystalline and durable condition.
- A hill, mountain or mound of resistant rock surmounting a peneplain.
- A geologic or topographic formation inclined at a single, consistent angle from the base.
- An accumulation of rock, boulders and debris transported by a glacier and finally deposited by it.
- Terminal or Recessional Moraine - Formed at the ice fronts, terminal moraines are "ridges of glacial drift, built at the former terminus of [a receding] glacier. They are made up partly of unassorted boulders, sand and clay, a deposit called till, and partily stratified gravel, sand and clay." (p 123)
- Ground Moraine - "The ground moraine is made up largely of till, but may contain small areas of stratified sand and gravel. The till is deposited in a broad sheet by the melting ice, so it is apt to be unstratified. [It] covers a much larger area than the terminal moraines, which are in long narrow belts." (p 255)
- A peak of resistant rock of sufficient height that it rises above the ice sheet like an island and thus escapes the effects of glaciation.
- Outwash, Outwash Plain
- "From the ice fronts, streams flowed away, leaving a sloping deposit called an outwash plain . . . and made up of rounded gravel and fine sand." (p 123)
- "A plain made by the wearing down of ancient mountains is usually spoken of as a peneplain that is, a region worn down nearly to a plain in a place where, formerly, there was rougher topography. The wearing down has been accomplished in a long period of time by the erosive action of streams and the weather." (p 34)
- A compact granular rock composed of quartz and derived from sandstone by metamorphism. Quartz is the mineral SiO2 (silica dioxide) that occurs in transparent and colored crystals and crystalline masses.
- Loose, granular materials resulting from the disintegration of rocks, consisting of particles smaller than gravel but coarser than silt. Sandy soil contains 85 percent or more of sand and no more than 10 percent of clay.
- A sedimentary rock consisting usually of quartz sand united by some cement (silica or calcium carbonate).
- A metamorphic crystalline rock which can be split along approximately parallel lines.
- Sedimentary (adj)
- Sediment is material deposited by water, wind or glaciers. Sedimentary rock is formed of mineral fragments transported from their source and subsequently consolidated and transformed by chemical action and pressure. Sedimentary rocks includes sandstone, shale (from clay, mud or silt), and limestone (from the accumulation of inorganic shells and skeletons of organisms).
- Loose sedimentary material composed of rock articles usually 1/20- millimeter or less in diameter. May also be used to refer to soil containing 80 percent or more of such material and less than 12 percent of clay.
- A trough of stratified rock in which the beds dip toward each other from either side.
- Unstratified glacial drift consisting of sand, clay and gravel with intermingled boulders.
- The shapes, patterns and physical configuration of the surface of the land, including its relief (local differences in elevation) and the positions of natural and man-made features.
Table Showing the Geological Column
||Period or Epoch
(Millions of Years)
||Age of man. Animals and plants of modern types.
||55 to 65
||Age of mammals. Possible first appearance of man. Rise and development of highest orders of plants.
|Age of reptiles. Rise and culmination of huge land reptiles (dinosaurs), of shell-fish with complexly partioned coiled shells (ammonites), and of great flying reptiles. First appearance (in Jurassic) of birds and mammals; of cycads, an order of palmlike plants (in Triassic); and of angiosperm plants, among which are palms and hardwood trees (in Cretaceous).
||135 to 175
||Age of amphibians. Dominance of club mosses (lycopods) and plants of horsetail and fern types. Primitive flowering plants and earliest cone-bearing trees. Beginnings of backbonedland animals (land vertebrates). Insects. Animals with nautilus-like coiled shells (ammonites) and sharks abundant.
||350 to 470
||Age of fishes. Shellfish (mollusks) also abundant. Rise of amphibians and land plants.
||Shell-forming sea animals dominant, especially those related to the nautilus (cephalopods). Rise and culmination of the marine animals sometimes known as sea lilies (crinoids) and of giant scorpion-like crustaceans (eurypterids). Rise of fishes and of reef-building corals.
||Shell-forming sea animals, especially cephalopods and mullusk-like brachiopods. Culmination of the bug-like marine crustaceans known as trilobites. First trace of insect life.
||Trilobites and brachiopods most characteristic animals. Seaweeds (algae) abundant. No trace of land animals found.
||First life that has left distinct record. Crustaceans, brachiopods and seaweeds.
||700 to 1000
||No fossils found.
The geological record consists mainly of sedimentary beds beds deposited in water. Over large areas, long periods of uplift and erosion intervened between periods of deposition. Every such interruption in deposition in any area produces there what geologists term an unconformity. Many of the time divisions shown above are separated by such unconformities.