In which one of the following folds is the axial plane found to be virtually horizontal?
- (a)Isoclinal
- (b)Anticlinal
- (c)Recumbent
- (d)Monoclinal
Correct — C, recumbent. A fold's axial plane is the imaginary surface that bisects the angle between its two limbs. As lateral compression intensifies this plane tilts further from the vertical — upright, then inclined, then overturned, and finally, when it is bent right over so that it lies almost flat, the fold is called recumbent. Both limbs of a recumbent fold then dip in the same direction at a low angle, which is exactly the geometry of a virtually horizontal axial plane.
- (a)Isoclinal — An isoclinal fold is defined by its two limbs being parallel to each other, so the axial plane and both limbs share the same (usually steep) dip — it is named for parallel limbs, not a horizontal axial plane.
- (b)Anticlinal — An anticline is simply an up-arching fold with the oldest rocks in its core; its axial plane is typically vertical or steeply inclined, not laid flat.
- (d)Monoclinal — A monocline is a one-limb, step-like flexure in otherwise horizontal strata — a single bend, without an axial plane turned to the horizontal.
Folds are classified by the attitude (dip) of their axial plane. The progression runs from symmetrical (vertical axial plane) to asymmetrical/inclined, to overturned (one limb pushed past the vertical), and finally to recumbent, in which the axial plane is bent over to near-horizontal. Recumbent folds form under intense lateral compression and are typical of the cores of great fold-mountain belts.
Read the question as 'which fold shows the most extreme tilt of its axial plane'. The sequence from upright to recumbent tracks increasing compression, and a 'virtually horizontal axial plane' is the end member — the recumbent fold.
- The axial plane bisects the angle between a fold's two limbs; its dip is the basis for classifying folds.
- Symmetrical fold: vertical axial plane; overturned fold: inclined axial plane with both limbs dipping the same way.
- Recumbent fold: axial plane bent over to near-horizontal — the product of intense lateral compression.
- Recumbent folds are common in the cores of Alpine- and Himalayan-type fold mountains, where nappes develop.
Increasing compression tilts the axial plane from vertical to horizontal; a horizontal axial plane = recumbent.
- Confusing a recumbent fold (horizontal axial plane) with an isoclinal fold (parallel limbs).
- Assuming a monocline has an axial plane in the usual sense — it is a single step-like bend.
Match a fold's geometry to its name — the discriminator is the dip of the axial plane.
No directly related past PYQ was found.
- practice — not a real PYQ
In which type of fold are the two limbs parallel to each other?
- (a)recumbent
- (b)isoclinal
- (c)monoclinal
- (d)symmetrical
Answer(b) isoclinal — its defining feature is limbs that lie parallel to one another.
- practice — not a real PYQ
The imaginary plane that bisects the angle between the two limbs of a fold is called the
- (a)limb
- (b)crest
- (c)axial plane
- (d)trough
Answer(c) axial plane — its dip is used to classify folds as upright, inclined, overturned or recumbent.