GEOLOGIC MAPS

Geologic Maps Shows geologic features - rocks and faults. Geology is represented by colors, lines, and special symbols unique to geologic maps.

Geologic Maps Geologic maps are broken into a series of areas that are called – Geologic Units. A geologic unit is a kind of rock of a given age range. A sandstone of one age might be colored bright orange, while a sandstone of a different age might be colored pale brown. Geologic units are named based on: where the rock was first studied - the Briones sandstone first described in Briones Valley,California. the kind of rock in the unit - 'Sandstone and shale,' 'Unnamed sandstone', or 'Undivided shale' Geologic units are named by the geologists who made the geologic map Geologic units are represented by different colors.

Geologic Maps

Letter Symbols Each geologic unit is assigned a combination of an initial Capital Letter followed by one or more small letters. The capital letter represents the age of the geologic unit. Eons, Eras, Periods, and Epochs, based on the fossils found in rocks. If the rock unit covers more than one time unit two capital letters are used. QT would indicate that the rock unit began to form in Tertiary time and was completed in Quaternary time. If the age is unknown, no capital letters are used. Small letters define the name of the unit or the type of rock. Kjm: Joaquin Miller sandstone formed in the Cretaceous Period Ks: unnamed unit of shale formed in the Cretaceous Period Gb: gabbro (a dark-colored igneous rock ) of unknown age.

Letter Symbols Kjm Ks gb

Lines on the Map Contact Lines Contact: Place where two different geologic units are found next to each other, Types of contacts: Depositional Contacts Faults Folds Solid, Dashed, or Dotted Lines Strike and Dip

Lines on the Map Depositional Contact The place where one type of rock rests on the rocks underneath. Shown as a thin line.

Folds & Faults

Lines on the Map Faults When different geologic units have been moved next to one another, the contact is a fault contact, Shown by a thick line. 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.

Lines on the Map Folds Marked on a geologic map with a line a little thicker than a depositional contact, but thinner than a fault. This line is referred to as a fold axis. Fold Axis. A line that follows the crest or trough of the fold is called the fold axis.

Lines on the Map Other Lines Solid Line: Places where the line is precisely located. Dashed Line: Contacts that are uncertain due to soil, vegetation, or human construction. The shorter the dash, the more uncertain the location. Dotted Line: Contacts that are covered by a geologic unit. Modified Lines: Lines on a map may be modified using symbols on the line (triangles, small tic marks, arrows, and more). For example, faults with triangles can identify the type of fault.

Lines on the Map Dashed Line Dotted Line Modified Lines

Lines on the Map Strike and Dip Shows tilted beds Consists of 3 parts: long line, short line, and number. Long Line: called the strike line, and shows the horizontal direction of the beds. Short Line: called the dip line, and shows which way the bed is tilted. Number: called the dip, shows the tilt of beds. The higher the number, the steeper the tilt, all the way up to 90 degrees if the bed is tilted all the way onto its side.

Map Key The map key defines the colors and symbols. Unit names and a short description of the kinds of rocks in that unit and their age are presented. Different types of lines on the map are explained, and the different strike and dip symbols. The map key will also include explanations of any other kinds of geologic symbols used on a map locations where fossils were found, locations of deposits of precious metals, location of faults known to be active, and geologic features specific to an area.

Map Key

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