## Geometry and Geology

From geology's start as a science, geologists have studied the 3D relationships of rocks.  The geologic map has evolved to depict these relationships.

The stereo net is a graphical depiction of 3D relationships on a flat page or computer screen.  The net may look like a map, but it should more closely be interpreted as centered on a particular point to show geometric relationships.  Like a map, however, the stereonet takes three dimensions and displays them on two dimensions.  Planes become curved lines, and lines become points.

Stereo Net Geometry

Getting the plane orientation from the net

• Solid lines are confidently located, while dashed lines are approximately located.
• Thick likes are faults, while thinner lines are the contacts between different rock units, either bedding planes or intrusive contacts for igneous rocks.

Definitions:

• Fault: a surface across which rocks have moved.
• Fault plane: initially, and over a short distance, the fault surface can be considered to be a plane.  Later deformations can deform the plane, and over long distances, faults cannot remain planar, but we can view the fault plane as the tangent at a point to whatever the true surface is.
• Bedding: the layering in sedimentary rocks, originally horizontal with rare exceptions (such as deltas or dunes which have cross bedding, and which the astute geologist will note).  Changing in bedding record deformation of the rocks.
• Bedding plane: initially, and over a short distance, the surface can be considered to be a plane.
• Strike and dip and formats

Last revision 8/19/2013