Stratigraphy is the study of the relationships between stratified (layered) rocks. These relationships form the basis of deciphering geologic history. Each layer of rock provides clues to the environment that existed when the layer was deposited. As rock types change from location to location this reveals differing environmental conditions that existed in these locations.

Many properties of the rock can be used to  provide a greater understanding of the environmental changes that have occurred in time and space. The properties of rocks that are used to help create a stratigraphic framework are :

Rock type - the word lithos means rock ,so this type of stratigraphy is called lithostratigraphy.

Fossils can show not only where features like beaches, or lagoons, or swamps may have occurred, but the fossils change through time and therefore can be used to determine when a particular layer of rock was deposited. Stratigraphic relationships based on the changes in types of fossils are called biostratigraphy.

The earth's magnetic fields are imprinted on certain types of igneous and sedimentary rocks. This is called paleomagnatism. The angle of the earth's magnetic field relative to the surface of the earth changes as one moves from the equator to the poles. It has also been found that this magnetic field has changed direction through time. This information can be used to both show where (the latitude) the rock formed, and when it formed. This type of stratigraphic analysis is called Magnetostratigraphy.

There are certain rules that can be used to determine when a body of rock was deposited relative to another body of rock. For example in an undisturbed sequence of sedimentary rock the oldest units would be at the bottom and the youngest units would be at the top. The type of stratigraphic analysis that can be performed by using this, and other, rules is called Chronostratigraphy.

Below is the framework of geologic time that has been built using stratigraphic relationships. The relationship between geologic time and the rocks and fossils found in several different areas is shown.

Time Period			Geologic Formations and Examples of Rocks (some formations cross the boundaries of geologic periods)				Examples Fossils from the Geologic Formations
Era	Age  (Millions of Years)	Period	Arkansas	Florida	Illinois	South Dakota (see map)
Quaternary	present  - 0.01	Holocene	.	.	.	.	.
	0.01 - 1.8	Pleistocene	.	Anastasia Miami Limestone Key Largo Limestone Bermont Fort Thompson Caloosahatchee	.	.	Shells - From the Bermont Formation Shells - From the Caloosahatchee Formation
Cenozoic	1.8 - 5	Pliocene	.	Caloosahatchee Tamiami Grey Limestone Bone Valley	.	.	Shells - From the Caloosahatchee Formation Shells and Sand dollars - From the Tamiami Formation Fossil teeth and vertebrate fossils - From the Bone Valley Formation
	5 - 26	Miocene	.	Bone Valley	.	.	Fossil teeth and vertebrate fossils From the Bone Valley Formation
	26 - 37.5	Oligocene	.	.	.	.	.
	37.5 - 54	Eocene	.	.	.	.	Fossil Fish (Herring)
	54 - 65	Paleocene	.	.	.	.	.
Mesozoic	65 - 136	Cretaceous	.	.	.	.	Tyrannosaurus Rex
	136 - 190	Jurassic	.	.	.	Sundance	Belemnites Othnielia Rex
	190 - 225	Triassic	.	.	.	.	.
Paleozoic	225 - 280	Permian	.	.	.	Minnekahta Limestone	Stromatolites
	280 - 325	Pennsylvanian	Brentwood	.	Mazon Creek	.	Fossil Plants and Animals from the Mazon Creek Formation
	325 - 345	Mississippian	Boone	.	Paint Creek	.	Blastoid
	345 - 395	Devonian	.	.	.	.	Trilobite (Phacops species)
	395 - 430	Silurian	.	.	.	.	Brachiopod  (Pentamerus) Orthoceras
	430 - 500	Ordivician	.	.	Galena-Platteville Group	.	Molds of Fossil Shells from the Galena-Platteville Group
	500 - 570	Cambrian	.	.	.	Deadwood	Trilobite (Ptychagnostus)
Pre Cambrian	570 - 4,600	.	.	.	.	.	.