3.5.1 Rock Units
Field geologists who study and map the bedrock that underlies an area of the land surface attempt to recognize rock units, which they can then represent on geologic maps. Rock types are not randomly arranged in the Earth’s crust but tend to exist in distinctive bodies called rock units. Rock units are large three-dimensional bodies of rock with compositions that are distinctive and different from adjacent rock units. Rock types vary widely in size, shape, composition, and origin.
Rock units are formed by the action of some particular process or set of processes—for example, sediment deposition, magma intrusion or extrusion, or metamorphism. Rock units can consist of sedimentary, igneous, or metamorphic rocks. The defining characteristic of a rock unit is that certain processes operate for some period of time to produce a body or mass of rock with fairly uniform rock type, or perhaps a consistent alternation of two or more rock types.
Here are some examples of the kinds of rock units a geologist might recognize in the field:
- A succession of sedimentary layers of one or several rock types, deposited in some distinctive sedimentary environment. The thickness of such a unit might range from several meters to many thousands of meters, and the lateral extent might range from hundreds of meters to a few hundreds of kilometers.
- A succession of volcanic rocks with distinctive composition. Such units might be as thin as a single lava flow (which could be as thin as a few meters) or as thick as many hundreds of meters. Such units might be interbedded with sedimentary rock units as well.
- A single igneous intrusive body. Such intrusive units cut across other rock units that they intrude.
- A unit of metamorphic rocks of a particular composition. Such a unit might have extremely complicated geometry, owing to intense deformation accompanying metamorphism. The intensity of metamorphism might vary systematically from one area to another within the metamorphic rock unit.
The minimum dimensions of a rock unit can be as small as meters or even decimeters, in the case of thin igneous dikes, for example. Units that small would not ordinarily be represented on a geologic map unless the purpose of mapping is to display the geology of a very small area in great detail, as for example at a major construction site.
Rock units are in contact with each other across three-dimensional surfaces or relatively thin zones of transition or gradation. The lines that represent contacts between rock units on a geologic map are the lines of intersection between the actual three-dimensional contact surfaces and the land surface itself. Recognizing and interpreting the nature of contacts between rock units is central to geological fieldwork. It’s largely by interpretation of the nature of such contacts that the geologic history of an area is worked out.
Many rock units receive formal names. The basic sedimentary rock unit (and also metamorphic and volcanic rock units) is the formation. Formations have two-part names: the first part is a place name, like a town, a river, or a mountain, and the second part is either the word “Formation” or a rock term like “Sandstone”. Volcanic and metamorphic rock units have similar two-part names. Formations can be subdivided into members, which can have either formal names or just informal names. Related formations can be lumped together into larger units called groups, which receive place names in the same way as formations. Intrusive igneous units, especially large units, can have formal names, but smaller units, even though they might be mappable, usually are not formally named.
3.5.2 Geologic Maps and Cross Sections
A geologic map is a map that shows the distribution of bedrock that is exposed at the Earth’s surface or buried beneath a thin layer of surface soil or sediment. A geologic map is more than just a map of rock types: most geologic maps show the locations and relationships of rock units.
Each rock unit is identified on the map by a symbol of some kind, which is explained in a legend or key, and is often colored a distinctive color as well. Part of the legend of a geologic map consists of one or more columns of little rectangles, with appropriate colors and symbols, identifying the various rock units shown on the map. There is often a very brief description of the units directly in this part of the legend. The rectangles for the units are arranged in order of decreasing age upward. Usually the ages of the units, in terms of the standard relative geologic time scale, is shown as well.
All geologic maps convey certain other information as well. They show the symbols that are used to represent such features as folds, faults, and attitudes of planar features like stratification or foliation. They have information about latitude and longitude, and/or location relative to some standard geographic grid system. They always have a scale, expressed both as a labeled scale bar and as what is called a “representative fraction”, 1:25,000 for example, whose first number is a unit of distance on the map and whose second number is the corresponding distance on the actual land surface.
All geologic maps (except perhaps very special-purpose maps that show all the details of an area that might be the size of a small room!) involve some degree of generalization. Such generalization is the responsibility of the geologist who is doing the mapping. Obviously, it is not practical to represent features as thin as a few meters on a map that covers many square miles: the width of the feature on the map would be far thinner than the thinnest possible ink line. The degree of generalization necessarily increases as the area covered by the map increases. You could easily see this for yourself if you have access to a geologic map of some small area together with the corresponding geologic map of the entire state: the detail of the small area on the state map would be much less than on the full map of that small area.
Most geologic maps are accompanied by one or more vertical cross sections, which are views of what the geology would look like in an imaginary vertical plane downward from some line on the land surface. These cross sections are constructed by the geologist after the map is completed. Their locations are selected so as to best reveal the three-dimensional nature of the geology. Cross sections are constructed by projecting downward the geologic features and relationships that are observed at the surface. Constructing cross sections requires the geologist to be able to visualize the geology in his or her mind. The degree of certainty about the geology shown on the cross section decreases downward with depth below the surface.