Fossils and relative dating
There are two basic approaches: relative age dating, and absolute age dating.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.
In either case, weathering and erosion are left free to work their mischief and erase some evidence of the geologic past. We’ll never know what happened during those 1.2 billion years? The Grand Canyon is not the only location with a rock record.
Scientists can correlate rock layers in different locations to get a more complete geologic record.
Unconformities can happen for a variety of reasons.
After all, a dinosaur wouldn’t be caught dead next to a trilobite.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.
In the Grand Canyon, for example, there is a layer of 1.7-billion-year-old igneous rock right under a layer of 550-million-year-old sedimentary rock.
What happened in the intervening 1.2 billion years? We just don’t, at the Grand Canyon, have a record of it.