Lower case letters that come after the capital letter(s), e. g., kjk, labeling a rock unit represent the rock unit's

Represent a rock type or rock unit

Explanation:

The alphabetical symbols are one of the important features that are used in the geological maps. It uses a combination of capital as well as small letter, where the capital letter is used in the front depicting the age of the rocks and one or more small letter is used behind that represents the type of rock or rock units.  

For example, Ks would represent the symbol for a shale rock which formed in the Cretaceous period.

Thus, the lower case letter that comes after the capital letters signifies the rock type (or rock unit).