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Geologic Maps

geologic map of part of the San Francisco Bay Area, California, USA
A geologic map of part of the San Francisco Bay Area, California, USA.

Letter Symbols

In addition to color, each geologic unit is assigned a set of letters to symbolize it on the map. Usually the symbol is the combination of an initial capital letter followed by one or more small letters. The capital letter represents the age of the geologic unit. Geologists have divided the history of the Earth into Eons (the largest division), Eras, Periods, and Epochs , mostly based on the fossils found in rocks. The most common division of time used in letter symbols on geologic maps is the Period. Rocks of the four most recent Periods are found in the San Francisco Bay area shown on this map, so most letter symbols begin with a capital letter representing one of the four Periods: J (Jurassic - 195 to 141 million years ago), K (Cretaceous - 141 to 65 million years ago), T (Tertiary - 65 to 2 million years ago), or Q (Quaternary - 2 million years ago until today).

Occasionally the age of a rock unit will span more than one period, if the period of many years required to create a body of rock happens to fall on both sides of a time boundary. In that case both capital letters are used. For example, QT would indicate that the rock unit began to form in Tertiary time and was completed in Quaternary time. The few geologic units formed an unknown amount of time ago have letter symbols with no capital letters.

The small letters indicate either the name of the unit, if it has one, or the type of rock, if the unit has no name. So Kjm (see 1 on map above) would be the symbol for the Joaquin Miller sandstone (formed in the Cretaceous Period), while Ks (location 2) would be the symbol for an unnamed unit of shale formed in the same Period, and gb (location 3) would be the symbol for gabbro (a dark-colored igneous rock ) of unknown age.

geologic map of part of the San Francisco Bay Area, California, USA geologic map of part of the San Francisco Bay Area, California, USA
geologic map of part of the San Francisco Bay Area, California, USA

Lines on the map

Contact Lines

The place where two different geologic units are found next to each other is called a contact, and that is represented by different kinds of lines on the geologic map. The two main types of contacts shown on most geologic maps are depositional contacts and faults.

All geologic units are formed over, under, or beside other geologic units. For example, lava from a volcano flows over the landscape, and when the lava hardens into rock, the place where the lava-rock rests on the rocks underneath is a depositional contact. Where the original depositional contact between geologic units is preserved, it is shown on the geologic map as a thin line (location 4).

geologic map of part of the San Francisco Bay Area, California, USA

Faults

However, in geologically active areas like the San Francisco Bay area, geologic units tend to be broken up and moved along faults (it is fault movements that cause earthquakes!). When different geologic units have been moved next to one another after they were formed, the contact is a fault contact, which is shown on the map by a thick line (location 5). Faults can cut through a single geologic unit. These faults are shown with the same thick line on the map, but have the same geologic unit on both sides.

Remember, just because the map shows a fault doesn't mean that fault is still active and is likely to cause an earthquake. Rocks can preserve records of faults that have been inactive for many millions of years. But knowing where the faults are is the first step toward finding the ones that can move. Special geologic maps of the faults known to be still moving are constantly being upgraded here at the United States Geological Survey, as well as by State geological surveys and university researchers.

geologic map of part of the San Francisco Bay Area, California, USA

Other Lines

Folds

Another kind of line shown on most geologic maps is a fold axis. In addition to being moved by faults, geologic units can also be bent and warped by the same forces into rounded wavelike shapes called folds. A line that follows the crest or trough of the fold is called the fold axis. This is marked on a geologic map with a line a little thicker than a depositional contact, but thinner than a fault (location 6).

geologic map of part of the San Francisco Bay Area, California, USA

Solid, dashed, or dotted lines

All thicknesses of lines are also modified by being solid, dashed, or dotted. Often contacts are obscured by soil, vegetation, or human construction. Those places where the line is precisely located it is shown as solid, but where it is uncertain it is dashed (location 7). The shorter the dash, the more uncertain the location. A dotted line is the most uncertain of all, because it is covered by a geologic unit, so no amount of searching at the surface could ever locate it (location 8). The lines on the map may also be modified by other symbols on the line (triangles, small tic marks, arrows, and more) which give more information about the line. For example, faults with triangles on them (location 9) show that the side with the triangles has been thrust up and over the side without the triangles (that kind of fault is called a reverse fault or a thrust fault). All the different symbols on the lines are explained in the map key (which is explained below).

geologic map of part of the San Francisco Bay Area, California, USA geologic map of part of the San Francisco Bay Area, California, USA
Continue to Strike and Dip
| What is a geologic map? | Colors | Letter symbols | Lines | Faults | Strike and dip | Map key |

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