Two or more Eras form an Eon, the largest division of geologic time.

A compilation of international stratigraphic standards that tries to reconcile all of the various geologic time scales into a single self-consistent whole.
, The International Commission on Stratigraphysets global standards for the fundamental scale for expressing the history of the Earth.

The Earth’s Geologic Time Scale Eons Eras Periods …

Just as the Mesozoic is divided into periods (, , and ), the Paleozoic Era is also divided.

Geologic time scale - Wikipedia

Four major divisions
Roughly, we can divide the time column into units comprehensible to us by saying the Cenozoic was the past time from present day back to the disappearance of the dinosaurs. The Mesozoic was the time that dinosaurs lived, the Paleozoic was before the dinosaurs with fossil plants and sea animals, and the Precambrian is the oldest group of rock: mysterious because so little is known about it. And get a load of this: Precambrian time occupies eighty percent of the earth's history!
On the previous page, we told you some of the active processes that assist the continents in their slow-motion dance around on the planet's surface, bumping and banging together (called plate tectonics), creating mountains when they collide. Volcanoes and erosion are also active forces that shape the land. Volcanoes bring up new material for land, erosion wears it down. That's part of the rock cycle.

Geologic Time Scale: Major Eons, Eras, Periods and Epochs

The table of geologic time spans presented hereagrees with the dates and nomenclature proposed by the International Commissionon , and uses the standard color codes ofthe United States Geological Survey.

Geologic time scale - definition of Geologic time scale …

A scale of time left by the rocks
At the left of this page as you scroll down is the time scale of geologic history. The numbers on the left-hand side of the table are millions of years before present time, and in each section of the table, the scale changes. That change means recent events recorded in the rocks are much better known than the older happenings of our earth, and the events of the very early world are mostly unknown. There are no fossils in the oldest rocks because there were no living things. In this geologic column, it is younger on the top than on the bottom, just like the piles of paper on my desk, or a layered sequence of sedimentary rock.

Eras, and Periods in the Geologic Column.

Pennsylvanian (286 - 320 MYBP)
Ouachitas - This is the time of South America, the Caribbean Plate, or another land mass bumping into us, causing the Ouachita Mountains to form. Some of the sediments were squeezed like toothpaste into mountains, while major thrust faults and reverse fault lines formed in the brittle units. The ocean over the area was getting more shallow. Rapid deep water clastic (rock fragment) sedimentation happened.
Arkansas Valley - There was rapid infilling with clastic sediments along the ocean basin growth faults.
Ozarks - The ocean that covered this area had shallow-water clastic sedimentation. Fault lines formed in a NE - SW direction, diagonally across the area.

Mississippian (320 - 360 MYBP)
Ouachitas - The ocean was deep early in this period. The slow rates of deposition in the Ouachita trough came to an end with the last beds of novaculite. Sedimentation became very rapid and the ocean started filling in.
Ozarks - Deposition and partial erosion of shallow water limestones, shales and sandstones.

Devonian (360 - 408 MYBP)
Ouachitas - 48 million years of slow deposition of deep water novaculite and shale in the ocean.
Ozarks - Slow deposition and erosion of thin units of shallow-water clastics and carbonates.

Silurian (408 - 438 MYBP)
Ouachitas - 30 million years of slow deposition of clastic sediment in the deep ocean.
Ozarks - Deposition and partial erosion of thin shallow-water carbonates.

Ordovician (438 - 505 MYBP)
Ouachitas - 67 million years of deposition of alternating sandstone and shale in the deep ocean.
Ozarks - Shallow seas covered the area, with deposition of layers of carbonates with minor clastics.

Cambrian (505 - 570 MYBP)
Ouachitas - The oldest formation that is exposed is the Collier Shale, and geologists don't know how thick it is.
Ozarks - Cambrian rocks are not exposed in north Arkansas. Where clastic and carbonate sediments were deposited form shallow seats on top of the Precambrian igneous basement.

Graphing the Geologic Time Scale

EOLOGISTS HAVE A warped sense of time. Maybe not as bad as astronomers do, but it is still mind-boggling to think in millions and billions of years.
As best that scientists can figure, the first rocks formed on the earth about 4.5 billion years ago. The earth is maybe 6 billion years old, but took it took a while for the crust to cool off and solidify. The rocks of the earth are the key in determining not only how the world is put together, but how old it is. You've seen places on the side of the road where the rocks lay in horizontal beds on top of each other. These are sedimentary rocks. In those parts of the world where the layers haven't been jumbled together by mountain-building forces, the bottom layers are older. Like a detective, scientists read the clues and can tell the ages of the rocks by the fossils in the layers. Since the 1950's, special techniques have been devised by geologists using radioactive isotopes present in rocks to accurately age date sedimentary, igneous, and metamorphic rocks. Guess what! The age dates correlate well with that of the fossil record found in sedimentary rocks, adding support to the acceptance of the Geologic Time Scale. So, by looking at road cuts, quarries and sections of rock cored by drilling, geologists pieced together a history of the planet.