Coming into this geology group as a biologist, I've been learning lots of new things! Many of the things that scientists of various disciplines learn from rocks are based on one concept, the law of superposition - this means that the oldest sediments are deposited first (at the lowest level), then newer ones deposited on top of them, then even younger ones, etc. When we look at horizontal layers of rock, the ones toward the bottom are older than ones towards the top. In order to "read" the rock story in the correct time order we read from the bottom up. This basic principal is the starting point for determining the relative age of rock layers.
Look closely at the rock section on the left and see how many layers you can identify. What characteristics of the rock change between the different layers? Which characteristics or features remain the same?
Of course, it isn't always this simple. Look closely at the rock section on the right and see how many layers you can locate. Many things may happen to the rock during or after deposition that may change the appearance and orientation of those original layers. Environmental conditions or physical events in earth's crust over time may even destroy some of the layers in some areas. The job of the sedimentologist is to interpret the tiny clues left behind in rock to understand both the environment in which the rock was formed as well as what has happened to the rock since. Learning to "see" what has happened to the sequence of rocks over time takes up a large portion of any geologist's training.
What can you think of that would make a good model for layers of rock deposited in a time sequence, with the oldest on the bottom and the youngest on the top?
One example that I thought of is a lasagna with alternating layers of noodles, sauce and cheese.