Geological Maps

maps representing the geological structure of some segment of the upper part of the earth’s crust. They are compiled from data obtained by means of geological surveying. They can also be made by processing materials accumulated in the course of geological research. Geological maps enable us to draw conclusions about the structure and development of the earth’s crust and the laws governing the distribution of minerals. They are used in planning exploratory and analytic work and in conducting geological engineering studies, construction work, and investigations dealing with water supply and land reclamation.

Depending on their content and purpose several types of geological maps may be distinguished—geological maps proper, maps of Anthropogenic (Quaternary) deposits, tectonic maps, lithologic maps, paleogeographic maps, hydrogeological maps, geological engineering maps, maps of minerals, prognostic maps, and geochemical maps.

The most important of these are geological maps proper (see the sample map on the insert facing page 257 in Bol’shaia Sovetskaia Entsiklopediia). These depict the age, composition, and origin of rocks, as well as the conditions of the occurrence of rocks and the nature of the borders between particular complexes, by means of a qualitative background (color and hatching) and letter, number, and other standard symbols. The color background serves to indicate the age of sedimentary, igneous, and metamorphic rocks. Hatching is used to show the composition of the rocks. Intrusive rocks and certain igneous rocks whose composition is conventionally depicted by color or letters are, however, an exception. There are also monochromatic geological maps showing both the composition of rocks and their age by hatching. All standard designations with their explanations are put in the map’s table of standard designations (its legend). The insert referred to above provides an example of the general color legend and the indexing of geological formations as recommended by the instructions on compiling and preparing for publication the USSR State Geological Map (published in 1969; scale, 1:200,000). These instructions make certain changes in the letter designations adopted earlier. Thus, new designations have been introduced in place of the indexes Pg (Paleogene), Cr (Cretaceous), Cm (Cambrian), Pt (Proterozoic), and A (Archean).

Horizontally occurring layers are depicted most simply on geological maps. The borders between layers are at an equal elevation, and their drawing on the map repeats the curves of the relief horizontals (see Figure 1). Where layers occur on a slope it becomes more difficult to depict them because the shape of their outcropping depends on the inclination of the

Figure 1. (a) Overall view of a hill and (b) its representation on a topographical map; (c) overall view of the hill and the sandstone and limestone layers that form it; (d) representations of the layers on a geological map

rocks and on the unevenness of the topography. The borders between layers on the map assume the form of winding lines that intersect horizontals (see Figure 2). Folded shapes of the bedding of rocks are designated on geological maps in the form of winding and closed contours. In this case anticlines are expressed by outcrops in the center of ancient layers, and synclines are expressed in the center of younger ones (see

Figure 2. (a) Overall view of a series of layers occurring on a slope and (b) representation of the layers on a geological map

Figure 3). Broken dislocations, such as faults, upthrusts, and overthrusts, are depicted on geological maps by a sharp shift of geological borders and by direct contact between strata of different ages along the joining surfaces (see Figure 4). Deep-lying crystalline rocks, such as granites and gabbro, forming intrusive bodies (batholiths, laccoliths, stocks) usually cut through contacts between layers of the strata containing them. The relationships in the bedding of intrusive and enclosing rocks are easily revealed on geological maps.

Geological maps of Anthropogenic (Quaternary) deposits reflect the distribution, age, composition, thickness, and origin of rocks of the Quaternary age. They show the borders of different stages of glaciation, sea transgressions and regressions, and the borders of the distribution of perennially frozen rocks. On geological maps proper, rocks of the Anthropogenic (Quaternary) age are preserved in those cases when they have their origin in seas or when they include mineral deposits (for example, placer gold and tin), as well as when their thickness is considerable and it proves extremely difficult to reconstruct the structure of the bedrock lying beneath the cover of loose deposits.

Lithologic maps serve to depict (usually by means of hatching) the composition and conditions of occurrence of rocks exposed on the surface or concealed under a cover of Quaternary beds.

Paleogeographic maps are constructed for a certain segment of time in geological history. They show the distribution of land and sea and indicate the composition of sediments or facies and their thicknesses.

Figure 3. A geological map depicting a folded structure, from (1) the youngest layers (in the center of the syncline) to (7) the oldest layers (in the nucleus of the antisyncline); below is a cross section along line AB

Geological engineering maps show, in addition to data on the age and composition of rocks, their physical properties—porosity, permeability, strength, and the like. They thus provide data necessary for planning projects.

Maps of minerals are compiled on a geological base on which signs and colors are used to show the groups of minerals (fuel, metallic, nonmetallic) and the particular types of mineral raw materials distributed over a given area. For each type of mineral the industrial and nonindustrial deposits and manifestations are distinguished. All direct and indirect signs of minerals are also included on the maps.

Prognostic maps reflect the laws of distribution of different types of mineral raw materials or their complexes. They are compiled on a geological base and offer a prospective assessment of elements of the geological structure of particular regions in reference to useful minerals. Taking into account the geological and economic conditions of each sector, these

Figure 4. A fold broken by a fault, X-X, from (1) the youngest layers to (5) the oldest layers; below is a cross section along line AB

maps reflect the degree to which deposits in sectors recommended for more detailed exploratory and analytic work have been substantiated and are reliable.

Geological maps are divided into four groups by scale: small-scale, medium-scale, large-scale, and detailed. Small-scale geological maps (1:500,000 and smaller) give an idea of the geological structure of the entire area of a particular region, state, continent, or the entire world. The geological map of the USSR on a scale of 1:2,500,000 (published in 1966) may serve as an example. Medium-scale geological maps (1:200,000 and 1:100,000) are compiled to depict the principal features of the geological structure of a territory and to provide a prognostic assessment of this area with regard to minerals. Large-scale geological maps (1:50,000 and 1:25,000) provide a more detailed depiction of the geological structure of regions that are promising with respect to deposits of minerals or that are designated for agricultural development or for the construction of cities, enterprises, or power stations. Detailed geological maps (1:10,000 and larger) make it possible to solve problems connected with the laws of distribution of ore bodies and with calculating reserves of minerals and the possibilities of industrial and civil construction. Medium-scale, large-scale, and detailed geological maps are accompanied by stratigraphic columns and geological cross sections.