Objectives - Students will be able to:

Explanation
Geologic maps are an extremely important form of communication for geologists.  The geologic map is a visual representation of geologic details. This is why field geologic mapping is both a science and an art.  On one hand, the mapper is recording objective observations.  On the other hand, the mapper must create a rendition of what they have observed.  The lines, symbols, patterns, colors, etc... on the map must be drawn with skill and precision, and must be clear and understandable to the reader.

Ideally the geologic map is created in the field from direct observations at close range.  This type of geologic map is sometimes called an “outcrop” or “bedrock” geologic map, because lines are drawn showing the actual perimeter of bedrock exposures at the surface.   Where there is more than one map unit in an outcrop, then a demarcation line, or “contact” is shown between them.  The location and nature of contacts provide extremely important information about the relationship of different rock types and the sequence of events which occurred.  Field geologic maps are typically drawn using a topographic base map, because geologic features can be more accurately located and more easily sketched when referenced to identifiable landforms in the area.

In some instances the map may represent interpretations of the geology which are made from evidence that has not been observed directly.  This type of geologic map is called an “interpretive” geologic map.  Indirect evidence may consist of aerial photographic evidence (including topographic or vegetation patterns), distant observations, geophysical data or geochemical data.  An interpretive geologic map therefore represents a generalization of the geology of an area.

Topographic Maps

Topographic maps are maps which show the topography, or shape, of the surface within an area, usually by means of contour lines.  A contour line is a line of equal elevation.  The vertical distance between contour lines is called the contour interval.  Steep areas will have contour lines close together, and gentle sloped areas will have contour lines far apart.  Where contour line crosses over a stream or gully on a topographic map, it forms a “V-shape”.  The V shape will always point upstream.  A contour line which forms a circle indicates the presence of a peak. 

Geographic Reference Systems:  Every map must have a geographic location reference system showing how the map area is related to the real world.  Most of the time, but not always, maps are drawn such that true north is towards the top of the map.  The magnetic declination for the area (angular measurement between true north and magnetic north) is stated or shown graphically somewhere on the map.  The reference system used is uses a “coordinate system” very similar to an X – Y graph.  The X – axis coordinate could be a longitude or “easting”.  The Y – axis coordinate could be the latitude or “northing”.  The pair of coordinates defines a point location referenced to a grid system and to other points on the map. 

The most common coordinate systems in use are:

Geographic Projections:   All maps of the earth’s surface are two dimensional representations of a three dimensional object.  In other words, the map is flat and generally rectangular, whereas the earth’s surface is irregular and generally rounded.  The round real surface has more actual area than can fit on the map, especially in the corner areas.  For this reason, the actual surface area must be distorted in some manner to make it all fit in the area of the rectangular map.  The transformation from the real surface to the flat surface is done mathematically, by changing the map coordinates.  This can be done in many different ways, each of which results in a different map “projection”.  There are many different projections used.  The projection used in the construction of the topographic map is specified somewhere on the map.

Geologic Maps

A geologic map is a map showing geologic information such as distribution, nature and age relationships of rock units, structural features, mineral deposits, and fossil localities.  As mentioned above, bedrock geologic maps show the distribution of actual bedrock outcroppings at the surface. Interpretive geologic maps show general distributions based on interpretive studies.  Geologic maps are often created on top of a topographic base map because the topography of an area often provides important clues about the relationship of map units. 

To create a geologic map, “map units” must be determined.  A map unit can be either a single lithology or a combination of lithologies which collectively are mappable or traceable in the field.  For example, a map unit might consist of a single layer of sandstone, or it may consist of a group of alternating sandstone and siltstone layers, each of which is too small to break out individually at the map scale which is being used.  In the field, the surface between two map units is called a “contact”.  On a geologic map, the contact between two map units is represented as a line.  The line, which can also be any shape, is formed where the surface between two map units intersects the ground surface.  The contact surface between the two units could be a normal sedimentary contact, a fault contact, and intrusive contact, or an unconformity.

Every geologic map should have a title and list the author and date.  Other essential information which should be present on every geologic map includes an explanation, a scale a north arrow, and a reference system.

Explanation:  The explanation is simply a list which explains the symbols used on the geologic map.  Most importantly, the explanation shows the break down of the map units.  Map units on the geologic map may be shown as different colors, different patterns, or they may be blank and contain only “unit designators”.  A unit designator is a label for a specified map unit, usually shown as a short abbreviation.  If the age of the rock is known, then the first part of the abbreviation (usually one or two letters) will indicate the age.  Following this will be a letter or letters which represent the type of rock.  For example, the unit designator “Pzgr” might be used to represent Paleozoic granite.

Scale:  The scale of a map shows the relationship between the length of a line on the map and the length of that same line in reality.  There are two types of scales in use.