Modifiers

[TyshawnB505]

Foundations of GMAT Verbal, 5th Edition
Hello,
I have a question about identifying a modifier. In chapter 6 page 101, in the Foundations of GMAT Verbal, the sentence reads as follows: Once rivaling the Greek Pyramids, an earthquake snapped the Colossus of Rhodes, a statue of the Greek god Helios, at the knees.

How do you determine that this is a modifier error? The book explains that after "Greek Pyramids, should be 'the Colossus Rhodes.'"

What should I be thinking about during the test to minimize this error on the GMAT exam?

Thank you for your assistance.

[RonPurewal]

"once rivaling the pyramids" must be followed by the thing that once rivaled the pyramids. in that position, "an earthquake" is nonsense.