Why does Scout describe the wait for Atticus to come home after the bush incident as "geological ages later" in To Kill a Mockingbird?

"Geological ages later" is a more poetic — and hyperbolic —way of saying, "That took a long time." You may also hear of things moving at a "glacial pace," which is another geological means of indicating an extremely long and slow process. Saying, "We didn't get in until geological ages later because the line was moving so slowly" is an example of using this in a modern context.

Geologic time is used to emphasize the extraordinary length, or duration, of an event or process. We often cannot comprehend the amount of time a geological process takes. For instance, the universe came into being about 13.7 billion years ago. Our Milky Way galaxy formed about 13.2 billion years ago. The difference between 13.7 and 13.2 may seem insignificant, but it accounts for 500 million years! Our solar system, which contains the sun, the Earth, and the rest of the planets, came into being around 4.6 billion years ago. This means it took about 8 billion years for our sun to form. The Earth itself is considered to be 4.5 billion years old. Thus, the earth took about 100 million years to form from the dust and rock orbiting the new sun. Scientists confirm that primitive, single-celled life forms were present on the earth 2.7 billion years ago. Early Humans evolved around 6 million years ago, but the modern form of humans evolved only around 200 thousand years ago. Human civilization itself is only about six thousand years old. Think about how long ago one hundred years seems. 1916 is very different from 2016. The average human lifespan is around 80 years. Thus, compared to life of the universe, the existence of a human would barely register on that scale.