The Sahyadri range is not straight but at many places it is curved, because

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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.  (c) 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).  (d) 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.

Concept: 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.

State PCS 2023 Q32 — Geography PYQ with Answer

State PCS 2023 Q32 — Geography PYQ with Answer