In the following illustration, PQRS is an exposed axial planar cleavage, striking north-south and dipping 40° east. B1 and B2 are the traces of two folded bedding planes on this axial planar cleavage. The fold can be classified as
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A plunging fold is characterized by an axial plane that is tilted and the fold axis that dips into the ground, as seen in the diagram where the bedding planes dip at 40°.
Step 1: Read the setup.
The cleavage plane PQRS strikes north south and dips 40 degrees east. The bedding traces B1 and B2 lie on this plane and we use them to name the fold.
Step 2: What the fold axis does.
The fold hinge follows the line where the bedding traces meet the cleavage. Because these traces are not horizontal, the hinge tilts down into the ground.
Step 3: Name that geometry.
A hinge that tips below horizontal is a plunging fold. The tilt of the traces is exactly this plunge.
Step 4: Reject the others.
A horizontal fold would need a level hinge, and upright or inclined describe the axial plane tilt, not the hinge plunge shown here. So those do not fit.
Step 5: Final choice.
So the fold is a plunging fold.
\[ \boxed{\text{Plunging fold}} \]
