Bihar’s geographical structure consists of rock systems. Match the rock systems with their descriptions :
a. Dharwar rock system
b. Vindhyan rock system
c. Quaternary rock system
d. Tertiary rock syst

Question

Bihar’s geographical structure consists of rock systems. Match the rock systems with their descriptions :
a. Dharwar rock system
b. Vindhyan rock system
c. Quaternary rock system
d. Tertiary rock system
1. Created by the rapid deposition of alluvium by Himalayan and Peninsular rivers
2. Can be found in the West Champaran district
3. Part of the oldest Archaean rock system
4. Sandstone, limestone, dolomite, quartzite and shale are the main constituents of this rock system
Select the correct answer using the codes given below.

UnlockIAS · Accepted Answer

Correct — A, a b c d 3 4 1 2, that is Dharwar–3, Vindhyan–4, Quaternary–1, Tertiary–2. Each pairing is fixed by the definition of the system itself, and three of the four need no Bihar knowledge at all. Dharwar takes 3. The Dharwar is India's classic Precambrian schist-and-greenstone succession, named from the country around Dharwad in Karnataka and carried on the Dharwar Craton — an Archaean continental block formed between about 3.6 and 2.5 billion years ago and reckoned the oldest part of the Indian peninsula, built of tonalite-trondhjemite-granodiorite gneisses, greenstone sequences and calc-alkaline granitoids. In the four-fold scheme Indian geography texts use — Archaean, Purana, Dravidian, Aryan — the Dharwars are the oldest metamorphosed sediments and are grouped with the Archaean basement, which is word for word what description 3 says. Vindhyan takes 4. The Vindhyan is a thick, essentially unmetamorphosed sedimentary pile, 'one of the largest and thickest sedimentary successions in the world', and its standard lithology is exactly the list in description 4: sandstone, limestone, dolomite, quartzite and shale. It is worth noticing that 4 is the only description that names constituents rather than an age or a place, and a list of sedimentary constituents can belong to a sedimentary cover and to nothing else on the card. In Bihar the Vindhyan is the south-western hill country — the Kaimur Range, the eastern arm of the Vindhyas, running about 483 km from around Katangi in Jabalpur district of Madhya Pradesh east to around Sasaram in Rohtas district of Bihar, north of the Son. Quaternary takes 1. The Quaternary is the most recent period of geological time, and in Bihar it is not rock in the ordinary sense at all but the deep alluvium of the Gangetic plain, laid down by rivers coming off the Himalaya in the north and off the peninsular plateau in the south. Description 1 describes a process of deposition rather than a composition, which is the giveaway. Tertiary takes 2, and the location is officially recorded: the Ministry of Mines told the Lok Sabha on 1 December 2021 that GSI had run a G4 reconnaissance survey for placer gold 'in parts of West Champaran district … in foothills of Siwalik Himalayas'. The Siwaliks are the youngest, outermost Tertiary belt of the Himalaya, made of sandstone and conglomerate that is the solidified detritus of the mountains to their north, with the rock generally dated between about 16 and 5 million years old — and West Champaran is the only district of Bihar that reaches them. One structural mercy is worth noticing: every one of those four pairs occurs in option (a) and in no other option, so a candidate certain of any single pair already holds the whole answer.

Why the other options are wrong — (b) This code reads Dharwar–1, Vindhyan–3, Quaternary–2, Tertiary–4, and it inverts the geological column almost completely. It makes the Dharwar the depositor of alluvium, when the Dharwar Craton is Archaean basement between about 3.6 and 2.5 billion years old and the alluvium under north Bihar is younger than two million years — a gap of three orders of magnitude. It then calls the Vindhyan Archaean, conflating a sedimentary cover with the crystalline floor it was laid on, and puts the Quaternary in West Champaran, which is true of the Terai plain in a trivial sense but is not what makes West Champaran geologically distinctive. Finally it hands the sandstone-limestone-dolomite-quartzite-shale list to the Tertiary, when that list is the textbook signature of the Vindhyan; the Siwalik Tertiary is sandstone and conglomerate, poorly consolidated, with no dolomite or quartzite in the definition.  (c) This code reads Dharwar–2, Vindhyan–3, Quaternary–4, Tertiary–1, and its central error is placing the Dharwar in West Champaran, at the far north-western tip of Bihar. West Champaran lies against the Himalayan foothills where the youngest rocks in the state are exposed and where the Main Frontal Thrust separates the Siwaliks from the Bhabar and the Terai; Dharwar-type basement in Bihar belongs to the opposite corner, the Chotanagpur fringe running south-east through districts such as Jamui, where the plateau's Precambrian rocks push north across the Jharkhand border. It repeats option (b)'s error of calling the Vindhyan Archaean, and then gives the Quaternary a hard-rock lithology of sandstone, limestone, dolomite, quartzite and shale, which is exactly what unconsolidated river alluvium is not.  (d) This code reads Dharwar–4, Vindhyan–2, Quaternary–3, Tertiary–1, and like the other two wrong codes it gets no pair right at all — none of (b), (c) or (d) scores a single correct pairing, which is itself the tell — a useful thing to notice, because on a four-item match list an option with zero correct pairs is usually built by rotating the whole key, and spotting one certain pair eliminates it instantly. It gives the Vindhyan's lithology to the Dharwar, which would mean the oldest metamorphosed rocks in India were undisturbed sandstone and shale; it puts the Vindhyan in West Champaran, three hundred kilometres from the Kaimur where Bihar's Vindhyan actually surfaces; it calls the Quaternary 'part of the oldest Archaean rock system', pairing the two extremes of the geological column; and it makes the Tertiary the depositor of the Gangetic alluvium, when the Tertiary Siwaliks are the source material the rivers cut into, not the deposit they built.

Concept: Indian geography teaches the country's rocks as four broad systems arranged by age, and this question is a straight test of that scheme. The Archaean or Precambrian basement, with the Dharwar succession sitting on and within it, is the oldest — the Dharwar Craton dates from about 3.6 to 2.5 billion years ago — and it forms the foundation of the peninsular shield and hosts most of India's metallic minerals. The Vindhyan is a vast, largely undisturbed sedimentary cover laid over that basement, mainly sandstone, limestone, dolomite, quartzite and shale; it supplies building stone and cement-grade limestone rather than metals, and its basin has yielded what may be the earliest known multicellular eukaryote fossils, possibly 1.6 to 1.7 billion years old. The Tertiary is the era in which the Himalaya rose, and its youngest, outermost belt, the Siwaliks, is a pile of sandstone and conglomerate shed from the rising mountains onto their own foreland — a range some 2,400 km long from the Indus to near the Brahmaputra and 10 to 50 km wide, bounded on the south by the Main Frontal Thrust with the Bhabar and the Terai below it. The Quaternary is the last two million years or so, represented across northern India by the alluvium of the Indo-Gangetic plain. Bihar is one of the few states in which all four are present within one boundary.

State PCS 2023 Q92 — Geography PYQ with Answer

State PCS 2023 Q92 — Geography PYQ with Answer