By studying a topographic map, you can interpret the landscape of a region. A unit whose top and bottom is readily recognized and which can be traced over a broad region is a geologic formation. A geologic map is a map on which the distribtuion of formations, as well as igneous intrusions and geologic structures, is indicated. Usually this is done by showing the trace of contacts between formations, and by coloring the formation or using a pattern to indicate the formation. These are a consequence of deformation.
Geologic maps can represent the orientation of a layer by a strike and dip symbol.
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.