MPSC 2023 Prelims Paper-I · Question 32 of 100

Correct — option (1). Read the stem carefully first, because it contains a negation that nothing on the page marks: the Sahyadri is NOT straight, and what has to be explained is why its crest line curves in and out instead of running as a clean wall from north to south. Begin with what the Sahyadri is. It is not a fold mountain thrown up by collision; it is the steep western edge of the Deccan plateau, an escarpment, which is why it presents an abrupt face to the Konkan and only a gentle slope on its landward side. That crest is also the water divide of the state. To its west the streams have a very short distance in which to reach the Arabian Sea, so they fall steeply; to its east the rivers set off on journeys across the whole peninsula. Now bring in the rainfall. The Konkan and the Ghat crest are among the wettest tracts in India, so the short western streams carry a large discharge down a very steep gradient, which gives them erosive power out of all proportion to their length. A river with power to spare erodes not only downwards into its bed but backwards at the head of its valley, wearing away the slope above its source and extending the valley upstream. That is headward erosion, and it is the only one of the three kinds of river erosion that can move a divide, because the divide is precisely the ground that the valley head is eating into. Each Konkan river is doing this independently, and they do not do it at the same rate: discharge, gradient and the resistance of the particular basalt flows all differ from one valley to the next. Where a stream has cut back vigorously it has gouged a bay into the crest; where the rock has held, the crest stands forward as a spur. The sum of many such advances and holdings is a crest line that curves — which is what the stem observes, and what option (1) explains.

• (2)The Sahyadri range is curved at many places because of the lateral erosion of the Konkan rivers originating in Sahyadri — Notice how this option is built: it is word for word identical with the keyed option except for a single adjective, and the whole question rests on knowing what that adjective changes. Lateral erosion is sideways cutting, in which a river attacks the outer bank of a bend and widens its valley floor; it is the process that produces meanders, flood plains and river cliffs, and it becomes dominant in the lower course where the gradient has slackened and the current is deflected from side to side rather than driven downward. Two things rule it out here. It belongs to the wrong part of the course, since the Konkan rivers are steep and youthful in the stretch nearest the Ghats, and more decisively it acts in the wrong plane: widening a valley leaves both its head and the divide above the head exactly where they were. Only erosion at the head of the valley can reshape the crest line.
• (3)The Sahyadri range is curved at many places because of the downward erosion of the Konkan rivers originating in Sahyadri — Downward or vertical erosion is a real and vigorous process on the Konkan rivers — with that gradient and that rainfall they cut deep, and the gorges, rapids and waterfalls along the western face of the Ghats are its work. This is the most defensible of the three wrong options for exactly that reason: the process named genuinely operates here. What it does not do is answer the question. Vertical erosion deepens a channel; it acts downwards from the river bed and its result is measured in the depth of the valley, not in the plan of the divide above it. A river could cut its bed to any depth without moving the crest line a single metre westward or eastward. Downcutting does steepen the slope at the valley head and so makes headward erosion easier, which is why the two are often seen together, but the process that actually consumes the divide is the one named in option (1).
• (4)Due to heavy rainfall the Sahyadri range is heavily eroded and hence it is curved — This option names the enabling condition and leaves out the agent. Heavy rainfall is indeed what makes the western face of the Sahyadri erode so fast, since it supplies the discharge that gives the streams their power, but rain does not carve a valley by itself; rivers do. The decisive objection is that this explanation cannot produce the effect the stem describes. Rainfall on the Ghat crest is heavy along its whole length, and a cause applied uniformly to a straight wall would wear that wall back uniformly and leave it straight. Curvature requires erosion concentrated at particular points and varying in strength between them, which is what a set of separate river valleys biting into the crest at different rates provides and what a general statement about rainfall does not. It is also the only option in the set that does not mention rivers at all, and in a question about the shaping of a landform the absence of an agent is worth noticing.

The Sahyadri belongs to a different family of landforms from the Himalaya, and the difference explains its shape. On the accepted account it is not a fold range at all but the faulted and eroded western margin of the Deccan plateau, left as a great scarp when the western edge of the peninsula subsided; the horizontal lava flows of the Deccan trap give the face its stepped profile, since harder and softer flows weather back at different rates. A scarp of this kind is under constant attack from the rivers on its steep side, and the way rivers attack is worth keeping distinct. Vertical or downward erosion cuts into the bed and deepens the channel, and it dominates the youthful upper course, producing V-shaped valleys, gorges, rapids and waterfalls. Lateral erosion cuts sideways into the banks, widening the valley and generating meanders, ox-bow lakes and flood plains, and it dominates the mature and old stages. Headward erosion cuts backwards at the head of the valley, lengthening the river upstream at the expense of whatever lies beyond its source. Only the third acts on a water divide, and its consequences can be dramatic: a vigorous stream cutting back through a divide may intercept the headwaters of a river on the other side and take them for itself, which is river capture or piracy. On the Sahyadri the same process, repeated by many short western streams working at unequal rates, drives the crest line slowly eastward and leaves it embayed and irregular in plan.

This is one of the seven questions in the paper whose stem is negative, and nothing on the page marks it as such — the words 'is not straight' are set in ordinary type and carry the whole item. Read at speed, the stem can easily be taken as asking why the range is straight, which inverts the question. The habit that protects against it is mechanical and cheap: before looking at any option, underline the verb and any negative attached to it, and say the demand back to yourself in your own words. The second thing to notice here is the shape of the option set. Options (1), (2) and (3) are identical sentences differing in one word — headward, lateral, downward — which is the commission's way of testing a definition rather than a fact. When three options differ by a single term, the question is asking what that term means, and no amount of general knowledge about the Sahyadri will substitute for knowing the difference between the three kinds of erosion. The fourth option, as often in this paper, offers a plausible general condition instead of a mechanism, and it is eliminated by asking whether the cause could produce the particular irregularity described.

• The Sahyadri is not a fold mountain range but the steep, faulted and eroded western edge of the Deccan plateau, which is why it presents an abrupt scarp to the Konkan and a gentle slope to the plateau behind it.
• The crest of the Sahyadri is Maharashtra's principal water divide: the streams west of it run short, steep courses to the Arabian Sea, while those east of it cross the peninsula to the Bay of Bengal.
• Headward erosion is the wearing back of a river at the head of its valley, which lengthens the valley upstream and drives the divide backwards; it is the only kind of river erosion that can change the position of a water divide.
• Downward or vertical erosion deepens a channel and produces gorges, V-shaped valleys and waterfalls, while lateral erosion widens a valley and produces meanders and flood plains; neither moves the head of the valley.
• The Konkan's very heavy rainfall and the steep, short courses of its rivers give them great erosive power near their sources, and because they cut back into the crest at unequal rates the Sahyadri crest line is left curved and embayed rather than straight.

• Missing an unmarked negation in the stem; in this paper the negative word is printed in ordinary type and nothing draws the eye to it
• Confusing headward with downward erosion, when only the first acts on the divide and the second acts on the depth of the channel
• Treating heavy rainfall as an explanation of an irregular landform, when a cause applied uniformly along a scarp would wear it back uniformly
• Assuming the Sahyadri is a fold mountain range like the Himalaya rather than the eroded edge of a plateau, which makes its scarp and its retreat hard to account for

Fluvial geomorphology appears in MPSC papers both in the abstract, as questions on the stage of a river and the landform associated with it, and locally, applied to the Sahyadri, the Konkan streams and the plateau rivers. The commission's favourite construction for the abstract version is the one used here: three options with identical wording differing only in the name of the process, which converts the question into a test of definitions. For the local version, expect questions on why the Ghats are steep on one side and gentle on the other, why the Konkan rivers are short and swift, why waterfalls are common along the western face, and how river capture has affected the divide. Negative stems are common across the paper and are not signalled typographically in this edition, so the first reading of any stem should establish whether the question wants the true statement or the false one.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following kinds of river erosion lengthens a valley in the upstream direction and can shift the position of a water divide ?

  • (a)Lateral erosion
  • (b)Downward or vertical erosion
  • (c)Headward erosion
  • (d)Deposition in the flood plain

  Answer(c) Headward erosion — it acts at the head of the valley, wearing back the slope above the river's source, so the valley grows upstream and the divide beyond it is consumed. Carried far enough it produces river capture, in which the advancing stream beheads a neighbour and takes its headwaters. Lateral erosion only widens the valley, vertical erosion only deepens it, and deposition builds land up rather than cutting it away.

• practice — not a real PYQ

  The Sahyadri or Western Ghats is best described as which of the following ?

  • (a)A range of fold mountains raised by the collision of two plates
  • (b)The faulted and eroded western edge of the Deccan plateau
  • (c)A chain of volcanic cones built along a rift
  • (d)A block mountain uplifted between two parallel rift valleys

  Answer(b) The faulted and eroded western edge of the Deccan plateau — it is an escarpment rather than a fold range, which is why it drops abruptly to the Konkan on one side and slopes gently towards the plateau on the other, and why its face shows the stepped profile of the horizontal Deccan trap lava flows. Fold mountains such as the Himalaya are built by collision and have a very different internal structure and profile.