Amateur naturalists are often confused by the distribution of fossils across our globe, wondering why certain fossils (of dinosaurs, for example) are plentiful in some areas but absent in others. In general, the fossils of plants and animals result from their remains having been trapped within sediment (lake deposits, ocean floor sediment, river mud, volcanic debris, etc.) that, over millions of years, hardened into sandstone, mudstone, limestone or some other sedimentary rock. Of course, most animals and plants die under circumstances in which their remains are consumed, undergo decay or are scattered by predators and natural forces before such fossilization can occur.
The type of fossils present in any region of our planet depends upon the age of the exposed sedimentary rocks in that area; rocks that formed from sediments that accumulated during any given geologic era will harbor fossils of life from that era. The exposure of these sedimentary rocks is a product of regional uplift, erosion and the deposition of overlying sediments (not necessarily occuring in that order). For example, Jurassic sedimentary rock, which accumulated during the Age of Dinosaurs, may have been lifted to the surface (as in large parts of the American West), may be buried deep beneath younger sediments or may have long-since eroded from the surface due to action of streams or glaciers; in other areas, these sediments may have never accumulated in the first place due to regional topography throughout the Jurassic Period.
Millions of years from now, Holocene sedimentary rocks will be explored by our super-human decendants or, perhaps, by visitors from other solar systems. Encased in those rocks will be the fossils of human civilization, including our domestic livestock, our cultivated plants, our pet poodles and the plants and animals that comprise our natural ecosystems. These Holocene sediments may be found deep in canyons, atop mountain ranges or outcropping from desert plains; in many areas they will lie deep beneath the fossilized remnants of younger civilizations while, in others, they will have already washed away to the sea.