MPSC 2022 Prelims Paper-I · Question 9 of 100

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.

• (1)(a) - (iv), (b) - (ii), (c) - (iii), (d) - (i) — 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.
• (2)(a) - (ii), (b) - (iii), (c) - (i), (d) - (iv) — 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.
• (4)(a) - (i), (b) - (iv), (c) - (ii), (d) - (iii) — 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.

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.

Fold types are a standing item in the MPSC physical geography block, and they are almost always set as a four-by-four match, because four names and four one-line descriptions fit neatly into the format. What the Commission is testing is whether a candidate has understood the classification or has merely collected its terms, and it builds the wrong rows accordingly: one row displaces every clue by one place, and another swaps only the two types that are genuinely alike. The method that answers such a question quickly is to find the clue that can belong to one item and no other. Here there are two of them, the description that mentions the ground and the description that speaks of one limb turned over the other, and either by itself identifies the correct row. Note also that this question prints no column headers, unlike the other matching items on the same pages; the two lists simply run alongside each other. That makes it even more important to remember that adjacency is a printing convenience and asserts no pairing at all.

• In a monoclinal fold one limb keeps a gentle inclination while the other is bent down steeply, so the beds form a single step; it is the extreme case of an asymmetrical fold.
• In an isoclinal fold compression has squeezed both limbs until they are parallel, dipping in the same direction and to the same degree.
• In an overturned fold the axial plane has been pushed past the vertical, so one limb has been turned over on to the other and both limbs dip the same way.
• In a recumbent fold the axial plane has been pushed round to the horizontal, so that both limbs lie parallel to each other and to the ground; further movement along a thrust plane produces a nappe.
• The types form a single scale of increasing compression — symmetrical, asymmetrical and monoclinal, then isoclinal, overturned, recumbent and finally nappe — and are best learnt in that order.

• Confusing the recumbent fold with the overturned fold, when the difference is whether the axial plane has merely passed the vertical or has been pushed round to the horizontal
• Accepting a matching row because its clues run in an orderly sequence, when a uniform displacement of every clue produces exactly that appearance
• Reading two side-by-side lists as though the printing paired them, which is a particular risk here because this question prints no column headers at all
• Learning the fold types as isolated definitions rather than as a scale of increasing compression, which is what makes them easy to interchange

Folds and faults reach MPSC papers as matching items joining a structure to its description, as one-line definition questions, and occasionally as questions asking which landform a named structure produces. The same four-by-four pattern is used for fault types and for the parts of a volcano, so the technique for answering matters as much as the content: find the description that admits of only one answer, pair it, and eliminate. Variants ask for the relation between fold type and mountain building, since the young fold mountains of the Alpine system contain the recumbent folds and nappes, and others ask about the economic significance of folds, because anticlines are the traps in which petroleum and natural gas collect. A single labelled sketch showing the sequence of fold types, with the axial plane drawn in each, answers all of them.

No directly related past PYQ was found.

• practice — not a real PYQ

  A fold whose axial plane has been pushed round until it is nearly horizontal, so that both limbs lie flat one above the other, is called

  • (a)an isoclinal fold
  • (b)a recumbent fold
  • (c)a monoclinal fold
  • (d)a symmetrical fold

  Answer(b) a recumbent fold — the name records that the fold is lying down, and the test is the attitude of the axial plane. In an isoclinal fold the limbs are parallel but still inclined, in a monoclinal fold one limb is gently inclined and the other steep, and in a symmetrical fold the axial plane is vertical with the two limbs equally inclined. Further movement of a recumbent fold along a thrust plane produces a nappe.

• practice — not a real PYQ

  Which of the following fold structures is most important as a trap for petroleum and natural gas ?

  • (a)The syncline
  • (b)The anticline
  • (c)The monocline
  • (d)The recumbent fold

  Answer(b) The anticline — an upfold of impermeable rock over a permeable reservoir bed holds oil and gas at its crest, with water below, which is why anticlinal structures are the first targets of petroleum exploration. A syncline is a downfold and collects water rather than hydrocarbons, while monoclinal and recumbent folds are classified by the attitude of the axial plane rather than by their value as traps.