Reading Comprehension

PT156 · S1 · P4 · Q22 Sand Fossils

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`Discovered in 1993, the site known as Ukhaa Tolgod, in the Gobi desert of Mongolia, is one of the world's best sources of fossils from the Late Cretaceous period, which ended about 65 million years ago.

Topic

Ukhaa Tolgod fossil site in Mongolia and how its fossils were so well-preserved.

Framework

Answer / Explain

Main Point

Scientists originally thought the fossils at Ukhaa Tolgod were preserved by sudden sandstorms, but evidence now points to rain-triggered debris flows (sandslides) in stable, vegetated dune fields as the real cause. (The Most Valuable Sentence appears at the end of the first paragraph and is echoed in the third: "this sandstorm hypothesis is probably mistaken" and )

P1: Introduction to Ukhaa Tolgod and the Original Sandstorm Hypothesis

This paragraph introduces Ukhaa Tolgod as an incredible fossil site from the Late Cretaceous and describes how the quality of the fossils was initially attributed to instant, deadly sandstorms that rapidly buried the animals. But new geological evidence challenges this idea.

P2: Geological Evidence: Types of Sandstone and Where Fossils Are Found

Here, we learn about three kinds of sandstone at the site. Only the third type—which isn’t windblown, has no layering, and contains large pebbles—holds the fossils. The other two types are related to wind activity but don’t have skeletons. This points away from the sandstorm theory.

P3: The Debris Flow (Sandslide) Hypothesis

This paragraph explains that the third sandstone type matches what you'd expect from a rain-soaked dune suddenly collapsing in a debris flow. Such an event could quickly bury animals just like an avalanche, explaining the fantastic fossil preservation.

22.

Which one of the following most accurately expresses the main point of the passage?

  1. Some scientists have long believed

    Wrong Emphasis: missing 'fossils'

    This answer doesn't reference the fossils we're trying to explain, so we should get rid of it without reading too deeply. The "geological formations" at Ukhaa Tolgod refers to the various types of sandstones, not the fossils. The OLD belief in this answer is that "the Gobi desert experienced lots of rainfall back in the past", whereas the OLD belief in the passage is that "the fossils were trapped by a sudden, violent sandstorm". This answer is doing the Last Sentence Trap, where it regurgitates something we were told about in the last sentence, since they know that sentence is still ringing in our ears.

    1% picked this

  2. Though the causes of rain-induced

    Wrong Emphasis

    Wrong Emphasis: missing 'fossils' Too Strong: settled on This answer doesn't reference the fossils we're trying to explain, so we should get rid of it without reading too deeply. Were we to read more closely, we wouldn't be able to sign off on the idea that "scientists have now settled on a particular explanation for sandslides" as a main point or even a supported claim.

    5% picked this

  3. Investigations of geological formations at

    Wrong Emphasis: missing 'fossils'

    This answer doesn't reference the fossils we're trying to explain, so we should get rid of it without reading too deeply. This answer, to its credit, does say something true, although it's not clear that the takeaway in the final sentence runs "contrary to previous assumptions". But the main point wasn't, "we think the Gobi desert was once a stable dune field". The main point was, "We're now thinking that these fossils weren't preserved by sandstorms, but rather by sandslides."

    22% picked this

  4. Correct

    Geological evidence suggests that the

    Why this is right

    This is the only answer choice that even mentions the central topic, which is this exceptional repository of fossils found at Ukhaa Tolgod. This conveys the fact that the passage was rejecting a previous explanation (sandstorms) and proposing a new tentative explanation (sandslides).

    Skill tested: Main Point · how this choice captures the passage's function is the move to repeat next time.

    69% picked this

  5. The search for fossilized dinosaur,

    Wrong Emphasis: missing 'fossils'

    This answer at first seems like it's talking about the extraordinary repository of fossils that is the central topic of the passage, but it's not. It's talking about the search for new fossils. The passage isn't about that at all. We're just trying to figure out how these fossils we've already found are in such good condition. Our previous theory was sandstorms, but now we're thinking sandslides, neither of which are mentioned in this answer.

    4% picked this

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