Geologic Map Units

The core data of any geologic map lies in the distribution of map units, each having distinctive physical characteristics. A fundamental map unit is a Formation, a rock unit with unique characteristics (such as color, texture, rock type, and/or age) that can be distinguished from other units in the field. A formation must be mappable, covering a large enough area to be shown on a geologic map. A lesser unit is called a Member, and several members combined can make a formation. Alternatively, several similar formations can be joined together into a larger Group, and groups can likewise be joined into a Supergroup. Rock formations are typically named after the locations (type localities) where the rock unit was first described or is best represented but can also refer to the composition of the rock unit.

Contacts delineate map unit boundaries. Solid line contacts mean that the location of the map unit boundary is relatively sharp or well-located. In contrast, a dashed contact may imply a gradational or less well-located boundary. Dotted contacts represent the inferred location of unit boundaries covered by overlying material. Other types of contacts depend on the geologic features present (like faults and intrusions). Contacts are not contour lines (which are related to elevation).

Map unit symbols

Map units always have labels or symbols to identify them. The exact symbols used on different maps can vary widely. Still, with most map unit symbols, the first letter is capitalized and typically represents the formation’s geologic age (Figure 10.4). In contrast, the second (lower case) letter describes the formation name, rock composition, etc. For example, the Clinton Formation has the map unit symbol St, where “S” is the age of the formation (Silurian period, 438-408 million years ago), and “c” is for Clinton (named for a town in New York where several well-exposed outcrops of these rocks can be found). The symbols are printed on the geologic map and in the map unit explanation.

Explanation of map units

All maps will have a description (names, ages, symbols, etc.) of the geologic map units, so the reader knows what the different units represent. Map units are listed from oldest (at the bottom) to youngest (at the top). Brief descriptions of the map units and special map symbols are commonly present. Look at the explanation if you see a symbol on the map you do not recognize!

Correlation of map units

Maps may include a diagram showing the age relations of the various map units.

Geologic Cross Sections

Maps may include one or more geologic cross sections. Topographic profiles that interpret the subsurface’s geology are inferred from the orientation of rock units at the surface). These figures can be highly interpretive but are very helpful in visualizing the distribution of rock units below the surface. The locations of cross-section lines or endpoints are marked on the map. Cross sections may be drawn with some vertical exaggeration better to show the changes in topography across the section line.

Other data

Other map data may include photo imagery, map location, data tables, stratigraphic columns, etc.

Using the following portions of the map, answer the questions: