The Law of Superposition states: the deeper it goes, the older it is. As we can see on the left side the age (and color) changes with depth. This way it is possible to track history yard by yard downwards. Sounds simple?

It should be, but sometimes even earth cheats on it and creates massive gaps, as can be seen on the right side of this image, taken in Taum Sauk, Missouri. The redish rhyolite rocks are 1.5 billion years old, while the gray dolomites above are 500 million years old. But there is nothing in between. There is a billion years of history missing here, people! How could earth manage to wait so long before slapping on a new layer? Geologists are literally graveled.