Physical Geology Slides-Geologic Maps

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Topographic Maps

Topographic Map Topographic maps show landforms and cultural features. This is a portion of a map of Germany. The grid squares are one kilometer on a side. Note the extremely high density of human cultural features.
Topographic Map A portion of a US topographic map (California). The grid squares are a mile on a side. Note the much lower density of cultural features.

Bench Marks

Bench Mark

Bench marks are the skeleton on which maps are supported. They are extremely accurately surveyed points to which the location of everything else on a map is related. This bench mark, on the Wisconsin-Illinois border, is the starting point for the entire survey grid of Wisconsin.
Bench Mark Timms Hill, the highest point in Wisconsin, is marked by the bench mark on the short pillar beneath the tower. The tower is there to allow resurveys of this and other key bench marks. Such resurveys correct errors on maps, detect movements of the crust, and detect subtle variations in the motions of the Earth.


Rock contact Geologists spend much of their time in the field trying to locate contacts between rock units, where the relationship between the units can be seen. This photo shows a contact between Ordovician dolomite in the top of the cut and Cambrian sandstone at the bottom.

What you don't know won't hurt you - it can kill you. About 1900 the citizens of Mindoro, near LaCrosse, got the idea of cutting through the ridge to shorten the distance to the main road by several miles. They thought it would be easy because the sides of the hill were soft sandstone. Nobody asked "if the rock is so soft, why is there a ridge here?" The ridge was capped with hard dolomite. The cut took two years and was the second-largest entirely hand-dug excavation in the United States.

Geologic Maps

small scale geologic map This is the Rolls-Royce of geologic mapping, published by the Ontario Geological Survey. Outcrops of rock are shown in a dark shade, concealed areas where a unit is inferred to exist is done in a light shade. The user knows exactly what the evidence is and where it is located. The prominent diagonal color boundary is a major crustal boundary called the Grenville Front.
regional geologic map Lots of small geological maps are combined into regional syntheses, like this one of the southwestern U.S. and part of Mexico. Maps of this sort can look misleadingly complete; there may be large areas with sketchy or no geologic map coverage, especially in parts of Mexico.
map of Pacific Ocean floor This map shows the geology of the Pacific Ocean floor. The materials on the sea floor are all pretty much the same but their age varies systematically, and the colors show the age of the sea floor. The small columns mark places where the sea floor has been drilled, and show the sequence of materials encountered. This map looks complete but there are areas of thousands of square miles that have never been investigated by a research vessel.
surficial material map This map shows only the materials on the surface in Wisconsin. Although it outlines the extent of the former glaciers very clearly, its real purpose is economic. This map provides clues to sand, gravel and clay resources, permeability of soil for septic systems, and so on.
geology of midwesr This map attempts to show everything. Where glacial deposits are thick, they are shown (mostly in brown), and where thin or absent, the underlying bedrock is shown.
tectonic map There are many kinds of specialized geologic maps. This map, called a tectonic map, shows the history of crustal movements in China and Mongolia.
gravity and magnetic maps Geophysics is the application of physics to the investigation of the Earth. These maps show variations in gravity and magnetism across Wisconsin due to buried rocks.
magnetic map The ancient rocks of South Dakota are deeply buried beneath younger rocks but show up on this magnetic map. Note the sharp break between NE-trending features on the east and S- or SE- trending features on the west. This marks a boundary between ancient rocks over 2500 m.y. old on the east and 1800 m.y. old rocks on the west. Occasional samples brought up from deep wells allow us to relate the magnetic patterns to the actual rocks.

geological and geophysical maps

On the left is a gravity map of Minnesota. In the center is a magnetic map. Note that the patterns are similar (this is a continuation of the NE-trending patterns on the South Dakota map.) On the right is a geologic map of Minnesota. Exposures are very scarce over large areas of northern Minnesota and much of that area was mapped based on the geophysical maps, rare outcrops, and occasional samples from wells.

Satellite Mapping

satellite image Satellite imagery is an important aid to geologic mapping. This is a Landsat image of the Green Bay area.

gravity map of ocean floor

This is a prototype of a revolutionary new kind of map: a map of the ocean floor made from space. Submerged mountains exert a slightly higher gravitational pull on the sea surface than average, and submerged depressions a slightly lower pull. Thus the sea surface is a subdued negative image of the sea-floor topography. Radar-mapping satellites can detect this sea-surface topography and thus map the sea floor. If I hadn't seen it, I wouldn't have believed it. (This is one of the earliest published maps of this sort - far more detailed maps have been done since.)

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Created 13 July 1998, Last Update 28 May 2003

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