LSAT 156, Section 1, RC Passage 4, Ukhaa Tolgod

RC Passage 4 Explanation

PASSAGE TEXT: Discovered in 1993, the site known as Ukhaa Tolgod, in the Gobi desert …

Paragraph Summaries

1. Ukhaa Tolgod has unusually well-preserved Late Cretaceous fossils. Scientists once thought sudden sandstorms preserved the animals, but new geological evidence suggests that explanation is probably wrong.
2. The site contains three types of sandstone. The wind-created deposits contain footprints but no skeletons, while all the vertebrate skeletons are found in a deposit that wasn’t made by wind.
3. The fossil-bearing sandstone looks like it was formed by sudden debris flows from rain-soaked sand dunes. These sandslides could have buried animals quickly and preserved their remains.
4. Clay buildup may have helped cause these sandslides by trapping rainwater in dunes. If sandslides preserved the fossils, the Gobi area likely had vegetation and rain, not just barren desert.

Analysis

It’s a classic science-y passage structure where the old explanation gets replaced by a better one.

The old theory: huge sandstorms buried the animals. Easy enough.

The new theory: not wind, but wet sand collapsing down dunes.

A key distinction is wind vs. not wind. If a deposit has windblown features but no skeletons, that’s bad for the sandstorm hypothesis. If the skeletons are all in the weird non-wind deposit, that points somewhere else.

In the third paragraph, there’s some mechanisms. It’s a chain: clay buildup requires stable dunes; stable dunes require vegetation; sandslides require rain-soaked dunes. So if the sandslide theory is right, the ancient Gobi wasn’t just an empty dry desert.

Be careful with any answer that says the passage proves the exact cause of the sandslides. It doesn’t. The author says that cause isn’t well understood. The passage gives a plausible mechanism, not certainty.