The symbol indicates the direction in which the layer tilts. A contact is the boundary surface between two formations, or between a formation and an intrusion. Developing the Concept of Geologic Time - So far in the class, we have used some fairly large numbers to describe the age of geologic features. The idea that the Earth is very old developed in the 18th and 19th century, accompanying the birth of the science of geology. This frame of mind set the stage for Darwin to propose his theory of evolution.
Geologists distinguish between two type of ages for a geologic feature.
Relative Age refers to the age of one feature with respect to another. Methods for determining chronologic age did not appear until the 1950s.