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Geologic Time Scale

The geologic time scale is divided into units and used by scientists to refer to times in Earth history.

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Geologic Time Scale

To infinity and beyond!

We can picture deep space, but what does deep time look like? If you divided up the 4.6 billion years of Earth history into one calendar year, as is done at the end of this lesson, you might get an idea.

The Geologic Time Scale

To be able to discuss Earth history, scientists needed some way to refer to the time periods in which events happened and organisms lived. With the information they collected from fossil evidence and using Steno’s principles, they created a listing of rock layers from oldest to youngest. Then they divided Earth’s history into blocks of time with each block separated by important events, such as the disappearance of a species of fossil from the rock record. Since many of the scientists who first assigned names to times in Earth’s history were from Europe, they named the blocks of time from towns or other local places where the rock layers that represented that time were found.

From these blocks of time the scientists created the geologic time scale (Figure below). In the geologic time scale the youngest ages are on the top and the oldest on the bottom. Why do you think that the more recent time periods are divided more finely? Do you think the divisions in the scale below are proportional to the amount of time each time period represented in Earth history?

License: CC BY-NC 3.0


The geologic time scale is based on relative ages. No actual ages were placed on the original time scale.

In what eon, era, period and epoch do we now live? We live in the Holocene (sometimes called Recent) epoch, Quaternary period, Cenozoic era, and Phanerozoic eon.

License: CC BY-NC 3.0


Geologic Time Condensed to One Year

It's always fun to think about geologic time in a framework that we can more readily understand. Here are when some major events in Earth history would have occurred if all of earth history was condensed down to one calendar year.

January 1 12 am: Earth forms from the planetary nebula – 4600 million years ago

February 25, 12:30 pm: The origin of life; the first cells – 3900 million years ago

March 4, 3:39 pm: Oldest dated rocks – 3800 million years ago

March 20, 1:33 pm: First stromatolite fossils – 3600 million years ago

July 17, 9:54 pm: first fossil evidence of cells with nuclei – 2100 million years ago

Precambrian and Early Paleozoic Era: Ancient Mountain Building and Iapetus Ocean Formation 
The recorded history (reflected in exposed rocks) of the Appalachian Mountains begins in the Proterozoic (fig. 12). A supercontinent formed during the mid- Proterozoic Grenville orogeny (mountain-building event) that consisted of most of the continental crust in existence at that time. This included the crust of North America and Africa. Sedimentation, deformation, plutonism (intrusion of igneous rocks), and volcanism associated with this event are apparent in the metamorphic granites and gneisses in the core of the modern Blue Ridge Mountains, west of Valley Forge (Harris et al. 1997). These rocks formed over a 100- million–year period and are more than a billion years old, placing them among the oldest rocks known from this region. They were later uplifted and thereby exposed to erosion for hundreds of millions of years. Their leveled surface forms a basement upon which all other rocks of the Appalachian Mountains were deposited (Southworth et al. 2001). Rocks of this age in the vicinity of Valley Forge include mafic to felsic gneisses, marbles, and metadiabases (Berg et al. 1980). 
The late Proterozoic, roughly 800–600 million years ago, brought extensional rifting to the area. The crustal extension created fissures through which massive volumes of basaltic magma were extruded (fig. 13A). This volcanism lasted tens of millions of years and alternated between flood basalt flows and ash falls. The volcanic rocks covered the Precambrian granitic/gneissic basement in southern Pennsylvania. Metamorphosed into greenstones, these rocks are exposed as the Catoctin Formation in central Maryland. 
The tectonic tension caused the supercontinent to break up, and a sea basin formed that later became the Iapetus Ocean (approximately 600–400 million years ago). This basin subsided and collected many of the sediments that would eventually form the rock units of the Appalachian Mountains (fig. 13B). Some of these sediments were deposited as alluvial fans, large submarine landslides, and turbidity flows (Southworth et al. 2001). Subsequent 
deposits of sands, silts, and muds covered the earlier sediments in nearshore, deltaic, barrier island, and tidal flat areas along the eastern margin of the continent (Schwab 1970; Kauffman and Frey 1979; Simpson 1991). Some of these sediments are present in the detrital sand, mud, and silt of the Cambrian-age Chickies and Harpers formations in the Valley Forge area (e.g., the western part of the park) (Berg et al. 1980; Wiswall 1993). Approximately 500 million years ago, the ocean basin deepened and the shoreline transgressed onto the continental margin, forming the depositional setting for chemical sediments (Bechtel et al. 2005). 
A carbonate platform, thicker to the east, provided the depositional setting for huge masses of carbonate, sandstone, and shale rocks during the Cambrian and Ordovician periods (545-480 million years ago) (Means 1995). Carbonate units such as the Elbrook and Ledger formations (underlying the area east and south of Mount Joy) and the Vintage Formation formed in this depositional environment (Means 1995; Bechtel et al. 2005). Cambrian to Ordovician units in the Valley Forge area, such as the Conestoga Formation (underlying the valley south of the park), record the locally intermittent transition from the carbonate platform to more nearshore terrestrial deposition associated with nascent uplift during the Taconic orogeny. 

November 18, 5:11 pm: Cambrian Explosion – 544 million years ago

Cambrian-Triassic Unconformity 
Valley Forge National Historical Park contains a unique geological feature called an angular unconformity that exists between the limestone-bearing Cambrian Ledger Formation and the mixed sedimentary strata of the Triassic Stockton Formation. The red sandstones and shales of the Stockton Formation dip gently to the north, whereas the layering within the Ledger Formation dips steeply to the south (Wiswall 1993). This tangible gap in geologic history spans more than 300 million years. This feature helped researchers relocate Bone Cave. The two rock types on either side of the unconformity are visible in the Port Kennedy quarry (Wiswall 1993). Early illustrations show red shale directly overlying the limestone of the quarry at the site of Bone Cave (see historic photograph in Daeschler et al. 2005). This red shale formed much of the sediment that had collapsed into the sinkhole uncovered during fossil excavation (Daeschler et al. 2005). The unconformity is mapped on the GRI digital geologic map derived from Berg et al. (1980) (Appendix A). Recent large-scale mapping by Kochanov (in prep.) details the nature of this feature (William Kochanov, Pennsylvania Bureau of Topographic and Geologic Survey, Senior Geologic Scientist, written communication, February16, 2010). 

Paleozoic Era: Appalachian Mountain Building

Taconic Orogeny

After 50 million years spanning the Early Cambrian through Early Ordovician periods, orogenic activity along the eastern margin of the continent began again (Means 1995; Bechtel et al. 2005). The Taconic orogeny (approximately 440–420 million years ago in the central Appalachians) was a volcanic arc to continent convergence. Oceanic crust and the volcanic arc from the Iapetus Basin were thrust onto the eastern edge of the North American continent, resulting in the closing of the ocean, subduction of the oceanic crust, creation of volcanic arcs, and uplift of the continental crust (Means 1995). Initial metamorphism of the deeply buried igneous and nearshore sediments into metabasalts, quartzites, and phyllites occurred during this orogenic event. The collision compressed originally horizontal layers of sedimentary rocks into folds and thrust them inland along large faults (Wiswall 1993).

The crust bowed downward to the west of the rising mountains in response to the overriding plate thrusting westward onto the continental margin of North America. This process created a deep foreland basin that filled with mud and sand eroded from the highlands to the east

VAFO Geologic Resources Inventory Report 27

(fig. 13C) (Harris et al. 1997). This Appalachian Basin was centered on what is now West Virginia.

The oceanic sediments of the shrinking Iapetus Ocean were thrust westward onto other deepwater sediments of the western Piedmont during the Late Ordovician. Sand, mud, silt, and carbonate sediment were then deposited in the shallow marine to deltaic environment of the Appalachian foreland basin. These rocks, now metamorphosed, currently underlie the Valley and Ridge physiographic province west of Valley Forge (Fisher 1976).

Acadian Orogeny

The Acadian orogeny (approximately 360 million years ago) continued the mountain building of the Taconic orogeny as the African continent drifted toward North America (Harris et al. 1997). Like the preceding orogeny, the Acadian event involved the collision of land masses, mountain building, and regional metamorphism (Means 1995). This event was focused north of eastern Pennsylvania.

Alleghanian Orogeny

During the Late Paleozoic and following the Acadian orogeny, the proto-Atlantic Iapetus Ocean closed as the North American and African continents collided. This event formed the Pangaea supercontinent and the Appalachian mountain belt we see today. This mountain- building episode is called the Alleghanian orogeny (325– 265 million years ago), and was the last major orogeny to affect the Appalachians (fig. 13D) (Means 1995). The rocks deformed during as many as seven phases of folding and faulting, producing the numerous folds of the Valley and Ridge province west of Valley Forge (Nickelsen 1983; Southworth et al. 2001). The strata of the Appalachian Basin (now the Appalachian Plateau province) remained relatively undeformed.

During this orogeny, rocks of the Great Valley, Blue Ridge, and Piedmont provinces were thrust inland along large faults and transported westward onto younger rocks of the Valley and Ridge province (Wiswall 1993). An estimated 20–50% (125–350 km, 80–220 mi) of crustal shortening took place (Harris et al. 1997).

Paleoelevations of the Alleghanian Mountains are estimated to have been approximately 6,100 m (20,000 ft) above sea level, analogous to the modern Himalaya Range in Asia. These mountains have been beveled by erosion to elevations less than 730 m (2,400 ft) above sea level west of Valley Forge (Means 1995). Erosion exposed the early Paleozoic formations, including the carbonates, following the Alleghanian orogeny (Bechtel et al. 2005).

Mesozoic Era: Rifting, Atlantic Ocean Formation, and Mountain Erosion

A period of rifting began during the late Triassic (230– 200 million years ago), as the deformed rocks of the joined continents began to break apart. The supercontinent Pangaea was segmented into roughly the same continents that persist today. This rifting episode

initiated the formation of the current Atlantic Ocean and caused many block-fault basins with accompanying volcanism to develop (fig. 13E) (Harris et al. 1997; Southworth et al. 2001). At this time, the uplifted Paleozoic units in the Valley Forge area, including the carbonate rocks, were lowered in the Gettysburg- Newark Rift Basin and reburied (Bechtel et al. 2005). Valley Forge National Historical Park is located near the southern edge of the Newark Basin (Wiswall 1993).

Extension and deposition were the dominant regional geologic processes during the Mesozoic Era. Thick deposits of unconsolidated gravel, sand, and silt were shed from the eroding Alleghanian Mountains. These sediments were deposited atop the much older Cambrian rocks at the base of the mountains as alluvial fans, in thicknesses that occasionally exceeded 5 km (3 mi). Throughout the basin, Triassic sandstone and red shale beds rest unconformably on the Cambrian-age units (Bechtel et al. 2005). The Triassic-age Stockton Formation (underlying the northern and eastern parts of the park), Hammer Creek and Brunswick formations, and Jurassic sediments record this depositional environment in the Valley Forge area. The rapid lithologic changes within the Triassic rocks indicate shifting depositional regimes that included nearshore, fluvial, lacustrine, and alluvial fan depositional settings (Sloto and McManus 1996). Deposition spread eastward to form part of the Atlantic Coastal Plain province. Thick layers of Cretaceous units such as the Patapsco Formation and Tertiary units such as the Pensauken, Bridgeton, and Bryn Mawr formations record this environment (fig. 13F) (Duffy and Whittecar 1991; Whittecar and Duffy 2000; Southworth et al. 2001). Only rare, scattered remnants of these units are present in the Valley Forge area. Exposed metamorphic rocks in the Blue Ridge province suggest that an immense amount of material was deposited. Many of the rocks exposed at the surface must have been at least 20 km (10 mi) below the surface prior to regional uplift and erosion.

Cenozoic Era: Karst Development and Ice Ages

The North American plate has continued to move westward since the breakup of Pangaea and the uplift of the Appalachian Mountains. The isostatic adjustments that uplifted the continent after the Alleghanian orogeny continued at a lesser rate throughout the Cenozoic Era (Harris et al. 1997). Over these millions of years, the still- buried carbonate rocks in the Valley Forge area underwent chemical weathering or dissolution along joints, cracks, fractures, or bedding planes. In some areas, these processes produced a subsurface network of interconnecting conduits and voids within the carbonate bedrock (Bechtel et al. 2005; William Kochanov, Pennsylvania Bureau of Topographic and Geologic Survey, Senior Geologic Scientist, written communication, February16, 2010). The network of solution cavities ranges in size from microscopic conduits to large, scenic caverns. Where dissolution approaches the surface, collapse of the overlying material may form sinkhole features such as Bone Cave (Port Kennedy Cave) (Bechtel et al. 2005).

28 NPS Geologic Resources Division

Quaternary-period erosion continues to create the present landscape. The Schuylkill River and its tributaries, such as Valley Creek, erode sediments from the lowering hillslopes and deposit alluvial terraces along the rivers (fig. 13F).

The Pleistocene masses of continent-scale ice sheets never reached the Valley Forge area, but terminated at 365 to 610 m (1,200–2,000 ft) above sea level in northwestern and northeastern Pennsylvania. Upland surfaces have been glaciated to rounded ridges and sand- and gravel-filled valleys in northern Pennsylvania. However, the colder ice-age climates affected the formation of the landscape at Valley Forge National Historical Park. The periglacial conditions that must have existed in close proximity to the ice sheets intensified weathering and other erosional processes (Harris et al. 1997). Landforms and deposits such as the Trenton Gravel are probably late Tertiary to Quaternary in age. These features were formed when a wetter

climate, sparse vegetation, and frozen ground caused increased precipitation to run into the ancestral river channels. This water flow enhanced downcutting and erosion by waterways and caused subsequent increases in downstream deposition (Means 1995; Zen 1997a, 1997b). Pleistocene fossil deposits, such as that in Bone Cave, record the climatic conditions and the impacts on local flora and fauna around 750,000 years ago (Bechtel et al. 2005).

Since the Pleistocene, the rivers and tributaries in the Valley Forge area continue to cut through thick layers of sedimentary deposits. Surface runoff erodes local slopes and is supplemented by mass wasting processes. Streams redistribute these sediments in riparian zones along the waterways and intermittently transport them toward the Atlantic Ocean. Natural erosion is augmented by human land disturbance activities such as agriculture, industrialization, drilling, quarrying, and widespread urban development throughout the area. 

December 1, 8:49 am: first insects – 385 million years ago

December 2, 3:54 am: first land animals, amphibians – 375 million years ago

December 5, 5:50 pm: first reptiles – 330 million years ago

December 12, 12:09 pm: Permo-Triassic Extinction – 245 million years ago

Triassic-age mudstones, siltstones, sandstones, and conglomerates underlie the northern highlands near Schuylkill River in gently tilted and folded beds. The Stockton Formation locally strikes east-west and dips 15° to the north. This unit unconformably rests on Paleozoic and Precambrian rocks (Sloto and McManus 1996). Some quarries within the park and surrounding region mark the contact between the Cambrian carbonates and the Triassic clastics (William Kochanov, Pennsylvania Bureau of Topographic and Geologic Survey, Senior Geologic Scientist, written communication, February16, 2010). Weathering and erosion of these sedimentary rocks created the rolling hills and narrow valleys characteristic of the Valley Forge area (figs. 9, 10) that made it an ideal location for the encampment. 
George Washington chose Valley Forge as an encampment site because it provided enough level area for the construction of quarters and facilities (fig. 11). The encampment was underlain by resistant geologic units (such as quartz-rich layers of the Chickies Formation) that formed high ground. This location was also easily defensible, bordered by Schuylkill River to the north and adjacent to Mt. Joy and Mt. Misery. Freshwater was provided by several streams and rivers within a small area. Dolostone underlies the broad, rolling upland along Outer Line Drive between park headquarters and the National Memorial Arch, whereas more soluble limestones underlie the valley at the base of a linear surface slope. This dolostone plateau at Valley Forge provided the Continental Army with a height advantage over enemy troops advancing from the south (Wiswall 1993). 

December 13, 8:37 pm: first dinosaurs – 228 million years ago

December 14, 9:59 am: first mammals -- 220 million years ago

December 22, 8:24 pm: first flowering plants – 115 million years ago

December 26, 7:52 pm: Cretaceous-Tertiary Extinction – 66 million years ago

December 26, 9:47 pm: first ancestors of dogs – 64 million years ago

December 27, 5:25 am: widespread grasses – 60 million years ago

December 27, 11:09 am: first ancestors of pigs and deer – 57 million years ago

December 28, 9:31 pm: first monkeys – 39 million years ago

December 31, 5:18 pm: oldest hominid – 4 million years ago

December 31, 11:02 pm: oldest direct human ancestor – 1 million years ago

December 31, 11:48 pm: first modern human – 200,000 years ago

December 31, 11:59 pm: Revolutionary War – 235 years ago

Source: http://www.timetoast.com/timelines/63215

See the video below for another analogy of geologic time:

Use the following link as a research resource:



  • The geologic time scale divides earth history into named units that are separated by major events in earth or life history.
  • Naming time periods makes it easier to talk about them.
  • Humans have been around for a miniscule portion of earth history.


Use this resource to answer the questions that follow.

Geologic Time - Introduction


1. How old is the Earth?

2. What is stratigraphy?

3. What is an eon?

4. What do eras and periods represent?

5. What is the lower limit of the Proterozoic era?

6. What are isotopes?

7. Why are isotopes important to geologic time?


1. Why do earth scientists need a geologic time scale?

2. Why are some units of the geologic time scale longer and some shorter?

3. How does the section that condenses all of geologic time into one year make you feel?

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