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.
