Question

Which of the following statements CORRECTLY describe(s) a parallel fold?

• A. Inner and outer arcs of the folded layer have same curvature
• B. Dip isogons are parallel to the axial plane
• C. Orthogonal thickness of the folded layer is constant
• D. Dip isogons converge towards core of the fold

Hint:

A parallel fold keeps constant perpendicular layer thickness, with dip isogons converging toward the inner arc; a similar fold keeps constant curvature and has isogons parallel to the axial plane.

Answer 1

The Correct Option is C, D

Step 1: Recall what a parallel fold is.
In Ramsay's classification of fold shape, folds are grouped using dip isogons, which are lines drawn between points of equal dip on the outer and inner surface of a folded layer. A parallel fold, also called a Class 1B fold, is defined by one key property: the layer keeps the same perpendicular, or orthogonal, thickness all the way around the fold, from limb to hinge.

Step 2: Recall the dip isogon pattern of a parallel fold.
Because the orthogonal thickness stays fixed, the inner arc (the concave side, or core, of the fold) must curve more tightly than the outer arc (the convex side). The dip isogons that join equal dip points on the two arcs are not parallel to each other, they converge, fanning inward and meeting closer together as you move from the outer arc toward the inner arc, that is, towards the core of the fold.

Step 3: Recall the contrasting Class 2, or similar fold.
A similar fold keeps the same wave shape and curvature on both the inner and outer arc, so both arcs look alike, and it is the thickness measured parallel to the axial plane that stays constant, not the orthogonal thickness. In a similar fold, the dip isogons run parallel to each other and parallel to the axial plane. This is the opposite behaviour from a parallel fold.

Step 4: Analyze option (A).
"Inner and outer arcs have the same curvature" describes a similar fold, where both arcs share the same shape. In a parallel fold the two arcs have different curvature, the inner arc being tighter, so this statement is incorrect for a parallel fold.

Step 5: Analyze option (B).
"Dip isogons are parallel to the axial plane" is again the signature of a similar (Class 2) fold, not a parallel fold. In a parallel fold the isogons are oriented closer to perpendicular to the layer boundary and they converge rather than running parallel to the axial plane. Incorrect.

Step 6: Analyze option (C).
"Orthogonal thickness of the folded layer is constant" is the exact defining property of a parallel fold, stated in Step 1. Correct.

Step 7: Analyze option (D).
"Dip isogons converge towards core of the fold" matches the isogon pattern described in Step 2 for a Class 1B parallel fold. Correct.

Step 8: Final conclusion.
The statements that correctly describe a parallel fold are that the orthogonal thickness is constant and that the dip isogons converge towards the core.
\[ \boxed{\text{Orthogonal thickness constant; dip isogons converge towards the core (C and D)}} \]