The letter symbols signify the name and age of the rock units in an area. The first letter refers to the geologic age, as shown above. The other letters refer to the formation name or the rock type. (To see what these units are, take a look at the geologic map of Rhode Island, where this comes from.)
A few of the age symbols are unusual; for instance, so many age terms begin with P that special symbols are needed to keep them clear. The same is true for C, and indeed the Cretaceous Period is symbolized with the letter K, from the German Kreidezeit. This is why the meteor impact that marks the end of the Cretaceous and beginning of the Tertiary is commonly called the "K-T event."
The other letters in a formation symbol usually refer to the rock type. A unit consisting of Cretaceous shale might be marked "Ksh." A unit with mixed rock types might be marked with an abreviation of its name, so the Rutabaga Formation might be "Kr." The second letter might also be an age term, particularly in the Cenozoic, so that a unit of Oligocene sandstone would be labeled "Tos."
All of the information on the geologic map, strike and dip and trend and plunge and age and rock unit, is won from the countryside by the hard work and trained eyes of geologists. But the real beauty of geologic maps—not just the information they represent—is in their colors. Let's have a look at them.
