Truly comprehending geologic time is an important step toward thinking like a geologist. This can be difficult to do because the earth is so old. If the entire history of the earth was laid out from the 50 yard line of a football field, the first "humans" (in the genus homo) would not be found until one inch before the goal line!

The geologic time scale can be thought of as the code to the tectonic and climatic history of the world. People as early as the ancient Greeks have made observations about geology. Philosophers and scientists from every century and many parts of the world have worked to put together the puzzle that is the earth's geologic history. This is still the overarching goal of geologic research today.

The current time scale formed with contributions from scientific discoveries in biology and chemistry, as well as from various disciplines of geology, including paleontology, paleomagnetism, and sedimentary stratigraphy. With future research, the time scale could continue to change. For instance, in 2009 the Tertiary period, a unit name that has been used since the 18th century, was split into the Neogene and Paleogene periods. This split was decided upon based on new research that showed some distinction between the two time periods.


What could be called the first geologic time scale was developed in the late 18th century and only had 4 geologic time (geochronologic) units. In the 18th and 19th centuries, European geologists started making detailed observations of fossils and rock similarities across areas of Europe. The locations where rocks were studied were often used to name new, geochronologic units that divided the time scale. Many of the names are still in use today, including: the Devonian, after the English county of Devon; Permian, after the state of Perm in Russia; and Jurassic, from the Jura Mountains in central Europe.

Today, the time scale is split into Eons (Phanerozoic and Precambrian) then Eras (Cenozoic, Mesozoic, and Paleozoic in the Phanerozoic Eon; and the Archean and Proterozoic in the Precambrian Eon). The geologic time scale figure shown here begins with the late Proterozoic Era. The earliest part of earth's history is lacking geochronologic detail compared to later time units. The Eras are split into Periods, each containing many Epochs, which are then split into Ages. This may seem like a lot of subdividing, but, Ages are often millions of years long.

Where do the names of the units come from? I mentioned that many periods are named after locations, but Eons and Eras have names that help to position them in time. The earliest Period in the Paleozoic is the Cambrian (the classical name for Wales is Cambria), so before this important time unit, we have the PREcambrian Eon.

The suffix -zoic means "relating to animals". So, the Protero-zoic is "before" or "early" animals. The Paleo-zoic represents an Era of "old" animals, the Meso-zoic is "middle" animals, and the Ceno-zoic is "recent" animals. The divisions between these Eras were determined with an obvious theme in mind.

Finding fossils in rocks is what initially lead to the creation of the geologic time scale. Certain fossils are found in the same rock units in many different parts of the world. This helps geologists correlate rock units across large distances based on the time periods in which these fossilized creatures would have been living. Even the earliest geologists have made observations on the types of fossils found in one rock unit versus the types in the rock units above or below it. This is the rock dating method called biostratigraphy. Fossils are often keys to finding out how much time passed between the deposition of some rock units and also in determining in what type of environment the rocks would have formed.

The division between the Precambrian and Paleozoic is marked by observations of an "explosion" of different fossil types found in the rocks of this age. During the Precambrian, important evolutionary events took place, including the evolution of multicellular organisms around 1 billion years ago, but not until 540 million years ago do rocks start to preserve diverse, hard-bodied fossils, such as trilobites. What caused this evolutionary diversification is constantly being studied and debated by scientists. Compared to the Precambrian, evolution since the Cambrian has been very rapid. This is reflected by the subdivisions in the geologic time scale. There are more subdivisions in the Phanerozoic than the Precambrian because younger rocks hold more age determining, such as biostratigraphic, information.

Many of the changes in geochronologic units correspond to mass extinctions. For instance, the infamous K-T boundary (the boundary between the Cretaceous and formerly named Tertiary Periods) is marked by an event that took place 66 million years ago. The currently accepted hypothesis for the extinction of non-avian dinosaurs at the end of the Cretaceous is that a large asteroid hit the earth, drastically changing our global climate.

Another extinction event marks the boundary between the Paleozoic and the Mesozoic Eras. The Permian-Triassic boundary is defined by an extinction event that killed 83 percent of all living genera on earth. Many other time divisions are based on fossil assemblage changes due to extinctions or diversifications of species resulting from major climatic changes.

You may notice that the time scale not only has names for each Era, Period, and Age; it also has a time range, typically in millions of years. These ranges were determined using radiometric dating techniques. There are a number of different techniques used for this, but the basic principle is the same.

Rocks contain minerals that contain chemical elements such as Uranium and Potassium. Certain isotopes of these elements undergo radioactive decay, or change from one element species to another over time. When this happens, a "daughter" element is created. The radioactive decay rate determines how quickly daughter elements are produced. Knowing this rate and the amounts of daughter and "parent" elements allows us to determine when this rock/mineral/element first formed. More information on radiometric dating can be found at

Radiometric dating of rocks helped determine that the earth is around 4.54 billion years old. Interestingly, this date came from meteorites and moon rocks. Based on research in the planetary sciences, the earth is the same age as the moon and other hard rock bodies in our solar system. The oldest rocks found on earth are found in Canada and are dated to be 3.96 billion years old. Unlike the moon, the earth's internal composition and chemical reactions cause plate tectonics on the earth's surface. With plate tectonics, our rocks are rarely left unaltered after they form. They are recycled, melted and reformed, many times. Therefore, finding a rock that formed on the surface of earth at the beginning of earth's history is practically impossible.

A lot of research and debate goes into refining the geologic time scale. A group of scientists called the International Commission on Stratigraphy (ICS website - aims to unify the geology community on time scale names and dates. One way they are doing this is by identifying rock units that best constrain accepted divisions between geochronologic units. The ICS's Global Boundary Stratotype Section and Point (GSSP's) markers are placed on rock outcrops around the world in order to mark important rock unit boundaries.

We have one of these GSSP markers at the Pueblo State Recreation Area. It marks the best place in the world to see the transition from Cenomanian Age to the Turonian Age of the Cretaceous Period (Figure). This location is special because of the detailed work done by local paleontologist W.A. Cobban on the fossils of ammonite species in the Greenhorn Limestone. In addition to biostratigraphic age constraints, we also have nearby volcanic ash layers that have been dated using radiometric techniques. The ICS's goal is for each Age subdivision to have one GSSP marker location associated with it, and the Pueblo marker is one of only 7 in North America. There are 101 worldwide as of 2012.

Cañon City has both very, very old rocks and young, not-quite rocks. The Royal Gorge is cut through 1.8 billion year old rock (Precambrian). This rock is some of the oldest in Colorado and has been the root of mountains, at the bottom of seas, and exposed at the surface many different times. There are millions of years missing in our local geologic stratigraphy (rock layers), but we have rocks from all the Eras and many of the Periods, including the Ordovician Harding Sandstone, the Pennsylvanian Fountain Formation, and the Cretaceous Pierre Shale.

The youngest rocks are those that have formed in the Cenozoic. Specifically, we have river deposits that were deposited during the Quaternary. These deposits may one day become rocks, but many are still unconsolidated, or loose soils and gravel.

So, don't take geologic time for granite (it's just too easy). Understanding what happened in our geologic past, helps us understand our non-renewable resources (where should we find the coal and oil-containing rocks?) and the effect global changes in climate can have on our living resources.