CDS (II) 2022 General Knowledge · Question 92 of 120

Correct — C, Mesozoic era. The Indian desert of western Rajasthan, called Marusthali, is now an arid country of longitudinal dunes and barchans with under 150 millimetres of rain a year, but the standard school account holds that it lay beneath the sea in the Mesozoic. The NCERT geography text puts it in exactly those words — it is believed that during the Mesozoic era this region was under the sea — and offers two pieces of evidence for it: the wood fossil park at Aakal, and the marine deposits around Brahmsar, near Jaisalmer. Marine sediments and fossil wood in the middle of a desert are what a former sea floor and a former coast leave behind. The Mesozoic runs from roughly 252 to 66 million years ago and covers the Triassic, Jurassic and Cretaceous periods.

• (a)Cenozoic era — The Cenozoic is the era we are still in, beginning about 66 million years ago. In India it is the era of the collision with Asia and the rise of the Himalaya and of the Indo-Gangetic trough — far too recent for the marine phase the evidence near Jaisalmer records.
• (b)Palaeozoic era — The Palaeozoic ran from about 541 to 252 million years ago and closed before the period the fossil and marine evidence points to. Choosing it means pushing the sea back one era too far.
• (d)Archaean and Pre-Cambrian era — These are the oldest rocks of all and make the crystalline basement of peninsular India. The Aravalli range that flanks this desert on its eastern side is indeed of that great age, which is what makes the option tempting — but the question asks about the desert lying under a sea, not about the age of the hills beside it.

Western Rajasthan sits on the edge of the peninsular block, and its rock record is layered rather than uniform. Ancient Precambrian rocks of the Aravalli system lie to the east; the Jaisalmer country to the west carries marine sedimentary beds laid down when a sea covered the area; and over all of it lies the loose sand and dune country of the present arid climate. The desert's own dryness is recent by geological standards, a product of the position of the Aravallis relative to the monsoon and of a rain shadow that starves the region of the summer rain that reaches the rest of the country.

The route into this question is the evidence and not the date. Ask what could possibly show that a desert was once a sea, and the answer has to be marine sediment and fossils — which is precisely what the school text cites near Jaisalmer. Then place that record between the two extremes on offer: it is younger than the Precambrian basement of the Aravallis and older than the Himalayan uplift of the Cenozoic, and the era in between is the Mesozoic. It is also the era in which the supercontinent Gondwana was breaking apart and the Indian plate began its journey north, which is why marine incursions across the western margin belong to that stretch of time.

• Marusthali is the name used for the Indian desert of western Rajasthan.
• The region receives below 150 millimetres of rainfall a year and carries longitudinal dunes and barchans.
• The school account holds that during the Mesozoic era the region was under the sea.
• The evidence cited is the wood fossil park at Aakal and marine deposits around Brahmsar, near Jaisalmer.
• The Mesozoic spans roughly 252 to 66 million years ago — the Triassic, Jurassic and Cretaceous periods.

• Answering with the age of the Aravallis instead of the age of the marine phase.
• Reaching for the Cenozoic because it feels like the era of Indian geography; that is the Himalaya's era.
• Treating the desert's aridity as ancient. The dune landscape is far younger than the rocks beneath it.

As a which-era item like this one, or through the evidence — a question naming the wood fossil park or the marine deposits and asking what they show.

• practice — not a real PYQ

  The wood fossil park at Aakal and marine deposits near Jaisalmer are cited as evidence for which one of the following?

  • (a)That the Aravallis were once volcanoes
  • (b)That the Indian desert was once under the sea
  • (c)That the Thar was once covered by an ice sheet
  • (d)That the Rann of Kachchh was once a freshwater lake

  Answer(b) That the Indian desert was once under the sea — the standard evidence for the Mesozoic marine phase of western Rajasthan.

• practice — not a real PYQ

  The Mesozoic era comprises which one of the following sets of periods?

  • (a)Cambrian, Ordovician and Silurian
  • (b)Triassic, Jurassic and Cretaceous
  • (c)Palaeogene, Neogene and Quaternary
  • (d)Devonian, Carboniferous and Permian

  Answer(b) Triassic, Jurassic and Cretaceous — roughly 252 to 66 million years ago.