Doubly Plunging Folds and Elliptical Outcrops

Elliptical outcrop patterns on a level surface are most commonly exhibited by doubly plunging folds.

Understanding Doubly Plunging Folds

• A doubly plunging fold is a fold where the fold axis plunges (dips) towards both ends of the fold from a central high point (in an anticlinal structure) or low point (in a synclinal structure).
• Imagine a wave shape that dips downwards towards both its left and right ends.

Reason for Elliptical Outcrops

• When a doubly plunging fold intersects a horizontal (level) geological surface, the outcrop pattern often forms a closed elliptical shape.
• The closure happens because the rock layers defining the fold plunge away in opposite directions from the center, eventually returning to or below the level surface.
• The outcrop shape reflects the geometry of the fold axis and the intersection with the horizontal ground.

Comparison with Other Fold Types

• Plunging Antiforms/Synforms: These folds plunge in only one direction. Their outcrop pattern typically narrows and terminates at one end, rather than forming a closed ellipse on a level surface.
• Non-plunging Folds: These have horizontal fold axes. They usually produce linear outcrop patterns parallel to the axis or V-shaped patterns if they are eroded through, not typically elliptical shapes on a flat surface.

Therefore, the characteristic elliptical outcrop on a level surface points towards a doubly plunging fold structure.