Detail from Bedrock Geology of Marathon County, Wisconsin
Elements commonly found on a geologic map
Map units
Map units indicate different types of rocks or other earth materials by a distinct color and symbol.
In the map shown above, different colored map units show the distribution of various rock types at the earth’s surface.
Contacts and faults
Contacts between different map units show where one kind of rock or other earth material passes into another. Sometimes the transition is clear (often depicted by a solid line) and sometimes it’s uncertain or approximate (a dashed line).
Faults are places where the earth’s crust has moved. On a geologic map, fault lines show where one unit has slid, been pushed up, or dropped down relative to the neighboring map unit.
In this example, a very deep part of a fault is exposed at the earth’s surface. In this case, there is a zone of deformed rocks rather than a single fault. This deformation zone is mapped as a shear zone, as indicated by the squiggly lines.
Strike and dip (orientation of rocks and faults)
Another way of remembering strike and dip is to think of the roof of a house. “Strike” is the direction the ridge of the house points, while “dip” measures the slope of the roof.
Strike and dip symbols show the orientation of rocks and fault planes. The strike, the long line, shows the direction the rocks point. The dip, designated by the short line and number, indicates the direction and angle that the rocks are tilted (90 degrees would mean that the rocks are completely vertical).
Cross section
A cross section represents a vertical slice of the earth’s crust showing the subsurface rock layers in much the same way that a slice of cake shows the layers of cake and frosting. Cross sections help the map user understand the arrangement of rocks beneath the earth’s surface.
Detail from Geologic Map and Cross Sections of La Crosse County, Wisconsin (Bulletin 101, plate 1).
