16.6: General Rules for Interpreting Geologic Maps

General Rules for Interpreting Geologic Maps

• Folds:
  • Anticlines have their oldest beds in the center, and their limbs (sides) dip away from the fold axis or hinge. Plunging anticlines plunge towards the closed end (nose) of the V-shaped outcrop belt.
  • Synclines have their youngest beds in the center, and their limbs (sides) dip toward the fold axis or hinge. Plunging synclines plunge toward the open end of the V-shaped outcrop belt.
• Streams:
  • They intersect dipping beds of rock, they will cut V shapes in the direction of dip.
  • They cut “V” shapes into horizontal beds and formation contacts that point upstream. The formation contacts are parallel to topographic contour lines, and the stream drainage system developed on horizontal and/or unstratified formations has a dendritic pattern that resembles the branching of a tree.
  • They cut across vertical beds, there will not be a V-shape
• Faults:
  • Upthrown blocks of faults tend to be eroded more (down to older beds) than downthrown blocks.
• General:
  • Geologic contacts migrate downdip upon erosion.
  • True dip angles can only be seen in cross-section if the cross-section is perpendicular to the fault or to the strike of the beds.