Match the following for types of folds :
(a) Monoclinal fold
(b) Isoclinal fold
(c) Recumbent fold
(d) Overturned fold
(i) Both limbs parallel but inclined to one side
(ii) One limb turned over the ot

Question

Match the following for types of folds :
(a) Monoclinal fold
(b) Isoclinal fold
(c) Recumbent fold
(d) Overturned fold
(i) Both limbs parallel but inclined to one side
(ii) One limb turned over the other limb
(iii) One limb inclined and the other perpendicular
(iv) Both limbs parallel to each other and the ground

UnlockIAS · Accepted Answer

Correct — option (3). Folds are classified by the attitude of their two limbs and of the axial plane between them, and each of the four descriptions in the second column names a distinct attitude. Start with the one that cannot be mistaken. Recumbent means lying down, and the only clue that speaks of the ground is (iv), both limbs parallel to each other and to the ground: a recumbent fold is one in which compression has pushed the axial plane over until it is nearly horizontal, so the two limbs lie flat, one above the other. So (c) pairs with (iv), and that pairing alone leaves option (3) standing. The overturned fold at (d) is the stage before that: the axial plane has been pushed past the vertical but not yet flattened, so one limb has been turned over on to the other and both now dip the same way, which is (ii). The monoclinal fold at (a) is the asymmetrical extreme in which one limb keeps a gentle inclination while the other is bent down steeply, and the clue that describes one limb inclined and the other perpendicular is (iii). The isoclinal fold at (b) is the one in which both limbs have been compressed until they are parallel, dipping in the same direction and by the same amount, which is what (i) means by both limbs parallel but inclined to one side. The completed pairing is (a)-(iii), (b)-(i), (c)-(iv), (d)-(ii), and it is printed at option (3). Note that the columns here carry no headers at all, only the two lists side by side, so their adjacency on the page is pure layout and settles nothing.

Why the other options are wrong — (a) This row gives the monoclinal fold the clue about both limbs lying parallel to the ground, which describes a recumbent fold, and it gives the isoclinal fold the clue about one limb turned over the other, which describes an overturned fold. The two errors are of the same kind: the row attaches to a mildly deformed fold the description of a severely deformed one. A monoclinal fold is a gentle structure, a step in otherwise horizontal beds where one limb dips steeply and the rest of the bedding stays nearly flat, and it belongs to the beginning of the sequence of increasing compression rather than to its end. Testing the recumbent pairing first would have removed this row immediately, since the recumbent fold is the one whose name states its own description.  (b) This row is wrong in every pair. It gives the monoclinal fold the description of an overturned fold, the isoclinal fold the description of a monoclinal one, the recumbent fold the description of an isoclinal one and the overturned fold the description of a recumbent one, so it moves each clue one place along the list. Rows of this construction are common in matching questions because a single displacement produces a row that looks orderly and is entirely wrong. The defence is to pair from the clues rather than from the pattern of the row: decide independently where the ground-parallel description must go and where the turned-over description must go, and any row that disagrees with either is finished, whatever shape it has.  (d) This is the row that punishes the commonest genuine confusion in the topic, between the recumbent and the overturned fold. It gives the recumbent fold the clue about one limb turned over the other, which sounds close enough to satisfy a candidate who has not fixed the difference. The difference is in the axial plane: in an overturned fold the axial plane has been tilted past the vertical, so one limb is inverted while the fold as a whole still stands up, whereas in a recumbent fold the axial plane has been pushed round until it is nearly horizontal and the whole structure lies on its side with both limbs flat. The row also gives the isoclinal fold the ground-parallel clue and the overturned fold the perpendicular-limb clue, so nothing in it survives a full check.

Concept: A fold is the response of layered rock to compression, and its parts have fixed names: the two sides are the limbs, the surface bisecting the fold is the axial plane, the line of maximum curvature is the hinge, an upfold is an anticline and a downfold a syncline. Folds are then classified by how far the axial plane has been pushed over, which is the same as saying by how much compression the rock has taken. A symmetrical fold has a vertical axial plane and limbs equally inclined; an asymmetrical fold has one limb steeper than the other; a monoclinal fold is the extreme of that asymmetry, one limb almost flat and the other bent down steeply, so that the beds make a single step. As compression increases the axial plane leans further: in an isoclinal fold both limbs have been squeezed parallel and dip the same way; in an overturned fold the axial plane has passed beyond the vertical so that one limb is inverted over the other; and in a recumbent fold it has been pushed round to the horizontal and both limbs lie flat. Beyond that the fold may break along a thrust plane and be carried bodily forward, giving the nappe structures that make up much of the Alps and parts of the Himalaya. The sequence from symmetrical through monoclinal, isoclinal and overturned to recumbent and nappe is therefore a single scale of increasing compression, and remembering it as a scale makes the definitions almost impossible to confuse.

State PCS 2022 Q9 — Geography PYQ with Answer

State PCS 2022 Q9 — Geography PYQ with Answer