You should be discussing rocks and not soil for the reasons Dr. A mentioned. Fossils are found in rocks.
There are 4 major divisions to
geologic time that are most commonly discussed (from oldest to youngest): Precambrian (4.6 Ga to 542 Ma), Paleozoic (542 Ma to 251 Ma), Mesozoic (251 Ma to 65.5 Ma), and the Cenozoic (65.5 Ma to present). Each of these in turn can be broken down into smaller and smaller divisions. They too can be grouped into larger divisions. Check out the link I provided.
You have to visualize geologic time like a time line. At the beginning, the earth was molten and then it cooled. Very little is known about earth's beginnings, but eventually the earth had oceans. Somewhere in *middle* you have the fish, plant, and dinosaurs flourishing. At the very end, you have humans.
The oceans are not simply H2O, but a chemical soup, albeit composed primarily of saline water. But in the precambrian, the earth was young and the chemistry of the oceans was far different than today. So was the atmosphere, for that matter. As a result of chemical reactions occurring in the ocean, sediment was formed and this sediment was composed of carbonate minerals, silica (SiO2), iron, and other compounds. This sediment would have fallen to the ocean floor and built up over time. While some think the sediment would eventually fill up the ocean basins, this is not true. The basins sink due to the weight of the sediment and this allows the sediment to get thousands of feet thick.
During this time, life developed and as this life died, it also sunk to the bottom of the ocean to be buried in the sediment. Through a variety of processes and pressure, the soft sediment hardens, or
lithifies, into rock. Eventually, life got more complex and continental land masses were formed, probably through volcanism. Volcanoes not only erupt ash, but lava as well. Both cool and both can form rocks. These land surfaces were weathered and eroded and formed soils and sediment of their own with their own chemical compositions. The sediment, similar to the sediment in the ocean, will also lithify over time. These are the Precambrian rocks. They are identified based on their fossil content,rock types (at times unusual), geochemistry, and their stratigraphic position located below rocks that contain abundant fossils displaying a great diversity life.
The Paleozoic includes the commonly discussed geologic periods, starting with the Cambrian and ending with the Permian. These sediments were laid down atop the older Precambrian sediments. It is entirely possible for the Paleozoic sediments to have been laid down on top of Precambrian
sediments (not rocks) and to have been interbedded with them. After all, the division between the Precambrian and the Paleozoic was based primarily on the explosion of life evident in the Cambrian rocks. This explosion of life also affected the composition of the oceans and the atmosphere -- it significantly increased the amount of free oxygen. This was enough of a change as to be noticeable in the rocks.
As you move up through geologic time, the Mesozoic, Cenozoic, etc., sediments kept piling up on top of the older sediments and rocks. At the same time, life was evolving slowly but surely into diverse life forms. Within the sediments, organisms were buried and eventually fossilized as the sediments lithified into rocks. The older rocks contain the oldest examples of life. As the rocks get younger, they contain a changing array of life.
The youngest rocks would be those from the Cenozoic since those were laid down last as far as Eras go. Right under your feet and in the deep ocean basins and in volcanoes, rocks are forming. These are the youngest rocks and sit atop all the other rocks.
Today, we are living in the Holocene Epoch, of the Neogene Period, of the Cenozoic Era, of the Phanerozoic Eon. Today you can see the future rocks on the beaches, sand dunes, active and erupting volcanoes, alluvial fans coming off the highlands, river bars and beds, and so on.
As to the other part of your post, dinosaurs are found in Mesozoic rocks because that's the time period in which they lived; so when they died, they were buried in those [Mesozoic] sediments. The Mesozoic sediments eventually became Mesozoic rocks.
Because hominids did not evolve until some 4+ million years ago, they were buried essentially at the top of the geologic rock record, in the Cenozoic -- and still in the Neogene Period in which we also live.
™ Version 4.2
(1)