The Earth has been aged at approximately 4.5 billion years.  This section will trace the geologic history of that part of the world which would eventually become the Tunkhannock Creek watershed.

    What would you have seen and experienced if you could have been present in that spot for all of Northeast Pennsylvania's history?

Pre-Archean, Archean, and Proterozoic (4.5 to perhaps 3.8 Billion Years Ago, (BYA))

    No reliable maps exist for the surface of the earth in its early stages.  Our planet was probably growing by being bombarded by meteorites, each helping to melt the surface.  Cooling magma began to sink and hotter magma rose to the surface, creating convection currents in the mantle which still exist today.  Small land masses called microplates, probably also the result of meteor impacts, "floated" atop the partially melted mantle and were carried along by these currents in the same way tectonic plates move today.

    Larger, continental land masses are thought to have formed by the accretion of many smaller microplates.  Laurentia, the predecessor of North America, is thought to have been a part of a larger continent, Rodinia.  Laurentia probably formed by accretion during the period of about 2.5 to 1.0 billion years ago, after meteor bombardment ended .

    During the Archean eon (3.8 to 2.5 BYA), life arose on earth as evidenced by fossilized bacteria.  During the Proterozoic eon (2.5-0.57 BYA), life on Earth diversified, and more complex fossilized life forms begin to show up, e.g., cyanobacteria (blue-green algae), protists (single-celled organisms), anthozoans (jellyfish) and worms.

(Click on the graphics to view a full screen image.)

Precambrian (650 Million Years Ago, MYA)

    Accurate maps of the world this long ago are uncertain due to lack of suitable fossils and other supportive evidence such as reliable paleomagnetism. Events occurring at this time include the "recent" (about 100 MYA) breakup of the first known supercontinent, Rodinia, and its subsequent split into several masses.  By the end of the period, the continental fragments were moving together again to form the new supercontinent, Pannotia.  Global climate was quite cold, with evidence of glaciation found on most continents. 

    About 650 million years ago, the future Antarctica was found near the equator, while Amazonia and Pennsylvania were near the South Pole.

Late Cambrian (514 MYA)   

    Pannotia began to break apart at the beginning of the Cambrian.  Ocean waters flooded the future North America and sediments were deposited atop older rocks in relatively warm, shallow seas.  Northeastern Pennsylvania, completely submerged at this time, would have been a zone of deposition of sandy and carbonate rocks, such as limestone (right).

    The "Cambrian explosion", a rapid diversification of life, occurred, leaving large numbers of fossils from organisms with hard, easily fossilized shells.

    Gondwana, the next forming supercontinent, took up position near the South Pole.




Middle Ordovician (458 MYA)

    With the breakup of Pannotia, the continents of Laurentia (North America predecessor), Baltica (Europe predecessor), and Siberia were separated by widening oceans.  Greenland was to be found on the equator.  The large continent of Gondwana was positioned over the South Pole, with parts extending northward to the equator.  Laurentia was found mostly in the southern hemisphere.  Much of it, including Pennsylvania, was underwater in a temperate sea.

    During the Middle Ordovician, tectonic movements caused ocean floor materials to be pushed onto the North American plate margin in the mountain building event known as the Taconic orogeny (right).  The weight of these mountains caused subsidence of the plate margin forming the long-lived Appalachian Basin to the west.  This basin eventually filled with sediments, first fine-grained, later sand and gravel.

    Life was becoming abundant in the seas.  Trilobites, brachiopods, bryozoa, corals, and graptolites were common.  The end of the Ordovician brought some of the coldest times in the history of the earth.

Middle Silurian (425 MYA)

    During the Silurian period, Laurentia, Baltica, and Siberia collide, closing the northern Iapetus Ocean and forming a land mass sometimes known as the "Old Red Sandstone Continent."  Elevated land masses were eroded.  Pennsylvania maintained its position just south of the equator.  Life flourished in the oceans, and many new species evolved.  Corals became abundant.  Fish evolved from primitive jawless, armored creatures into scaled, jawed animals.  Terrestrial plant life began to flourish.

Early Devonian (390 MYA)

    During the Devonian period, the landmasses of Avalonia and Baltica (Europe) collide with North America in the Acadian orogeny, lifting the Acadian Mountains to the east of Pennsylvania.

   Now raised above sea level, the Acadians began to erode, supplying sediments to the Appalachian Basin to the west (left).  At first the sediments were fine-grained, becoming the shales and siltstones that are now exposed in our area.  This basin filled, subsided, and continued to fill, pushing the coastline farther to the west.  Eventually the marine sediments were topped by sediments deposited in freshwater environments.  During the late Devonian, vertebrates moved onto land in the form of amphibians.  Plants and insects diversified in these terrestrial environments.


Carboniferous (365-290 MYA)

    The name Carboniferous (carbon-bearing) is sometimes used to include the Mississippian and Pennsylvanian periods.  It was during this time when the great coal fields of Pennsylvania were being formed.  At this time, Pennsylvania was on or near the equator and had a tropical climate, perhaps like the tropical rain forests of today.  The sea encroached into Pennsylvania from the west and the coastline moved back and forth across the state.  Trees, ferns, amphibians, insects, and early reptiles occupied the land.

Late Permian (255 MYA)

    Most of the land masses coalesced into the supercontinent, Pangea, a name which means "all land."  In the early Permian, the land regions near the South Pole became glaciated.  The evidence of this glaciation found in southern South America, southern Africa, Antarctica, Australia, and the Indian subcontinent was useful in confirming the theory of plate tectonics.

    The Permian was a time of major change for the east coast and Pennsylvania.  Africa crashed into North America raising the Alleghanian Mountains (referred to as the Central Pangean Mountains on the graphic to the left) during the Alleghanian orogeny.  Erosion immediately began, and sediments were shed west to the Appalachian basin (right).  By the end of the Permian, the once great mountains had lost much of their impressive height.  Although still located near the equator, Pennsylvania no longer enjoyed a moist climate, but became a desert, the result of being in the rain shadow of the mountains and located in the middle of the continent.

    The end of the Permian was also a time of major extinction, especially of shallow water organisms.  A variety of hypotheses have been proposed to explain this great extinction, ranging from meteor impact to loss of shallow water habitats due to continental collisions.  Trilobites are one group that disappeared.  On land, reptiles were proliferating.

Early Triassic (237 MYA)

    The Mesozoic Era ("middle life") includes the Triassic, Jurassic, and Cretaceous periods, and covers the time of 250-67 MYA.  In the Early Triassic, Pangea was still relatively intact, and most land masses were contiguous, allowing for free exchange for terrestrial organisms.  North America's gradual rotation swung Pennsylvania north of the equator as it began the trek to its current location.  Although the climate was still warm, precipitation was probably quite variable and seasonal.  This was a prolonged time of erosion for Pennsylvania, as it was completely above sea level, and there was no further deposition.  During the Late Triassic, Africa and North America began rifting apart, a process that would continue into the Jurassic.


    Lasting about 68 million years, the Jurassic extended from  208-140 MYA.  Although little is known about events occurring in Pennsylvania at this time, earth-shaking events were taking place on the global stage.  As North America pulled away from Africa, the infant Atlantic Ocean was born.  At the same time, the Gulf of Mexico began to open.  Between the time these two 'snapshots' were taken 42 million years transpired!

Late Cretaceous (94 MYA)

    During the Cretaceous, the separation of continents continued as a result of seafloor spreading.  The North Atlantic widened as Laurasia split into North America and Europe.  The South Atlantic Ocean widened as Gondwana fragmented.  India sped towards Asia.  The Rockies began uplifting.

    Global climate was significantly warmer than today.  Dinosaurs were dominant life forms, extending nearly from pole to pole.  Both mammals and birds made their first appearances.  At the end of the Cretaceous 66 MYA (commonly known as the K-T boundary), many species became extinct.  The predominant explanation for this is a meteor strike at the site known as Chicxulub (indicated on the map on the right with a bullseye), on the Yucatan peninsula of Mexico.  It is thought that the meteor hit the earth obliquely and sprayed debris mostly into North America.

Cenozoic Era (67 MYA-present)

    The Cenozoic Era is divided into the Eocene (67-2 MYA) and the Quaternary (2 MYA-present.)

Middle Eocene (50 MYA)

    Some of the world's landmasses were moving quickly during the Eocene.  After racing at "breakneck speeds" of perhaps as much as 15 to 20 cm per year, India crashed into Asia pushing up the Tibetan Plateau and the Himalayas.  Australia, breaking away from  Antarctica, also began racing northward on a collision course with Indonesia.

    During the Eocene, the climate remained warm, contributing to intensified chemical weathering, which converted Pennsylvania's solid rocks into materials like clay. This lead to a thick covering of weathered materials over the bedrock.

Middle Miocene (14 MYA)

    The earth began to cool, yet was still warmer than today.  Sea level was higher than in modern times.  Peninsular Florida was flooded, as was much of southern Europe.  The Panama land bridge, connecting North and South America, was under water, putting an end to the free exchange of terrestrial animal life.

    However, conditions are changing.  Continent-continent collisions in various parts of the world continue to form several mountain ranges, in a major round of orogeny.  The Pyrenees, Alps, and Himalayan Mountains continue to grow.  As continental crust is crumpled into smaller areas, the ocean basins will enlarge slightly, lowering sea level and beginning the next great cooling.

    Pennsylvania experienced extensive erosion, which may have lowered the land surface of the eastern part of the state by several hundred feet.  The current drainage patterns of surface water were fairly well established during this time, delineating our current drainage basins and dissecting the landscape into much of what we see today.

    During the Cenozoic, the modern assemblage of species was evolving.  Of special importance was the appearance and diversification of grasses and the now-dominant mammals.

Last Glacial MaximumLast Glacial Maximum (18,000 YA)

    During the Pleistocene Epoch, the most recent Ice Age occurred, but the whole world was not frozen solid!  Although the Antarctic and Arctic ice sheets were much more extensive than today, generally, only the northern portions of the United States were ice covered.  Surprisingly, large areas of Alaska and Siberia were not!

    The oceans were the source of the water that froze, and sea level was lowered by hundreds of feet.  Newly exposed coastal ocean bottom added to the size of continents.  At this time, the now-coastal city of New York would have been hundreds of miles inland!

    Because land masses have a higher albido than oceans, more heat is reflected back into space.  This leads to a cooling of the earth and more freezing.  The positive feedback mechanism will push the climate into greater and greater cooling.

    At this time, only the northeastern and northwestern corners of future Pennsylvania were glaciated.  The entire Tunkhannock Creek Watershed atlas area was included in the area of glaciation.

The Current WorldThe Present

    During the short time span of an individual human life, the continents may seem to be quiet and static.  This is deceptive, however!  We know that they are in motion and can measure their rates of movement.  

    Climatologists debate whether we are headed into or out of an Ice Age.  The Sangamon interglacial may be a minor hiccup before we head back into another deep freeze.

    On the other hand, might global climate be warming due to the input of greenhouse gases of anthropogenic sources?  Sea level is measurably on the rise.  This shrinks the size of land masses, which can set in motion a positive feedback system of further warming.  Land masses have a greater albido than water, reflecting more insolation back to space.  With less reflectance, oceans absorb sunlight, and the earth will warm.  How much? No one knows, but at times in the past the earth has been significantly warmer than it is now.

    Some have suggested that the earth will warm, melting the ice caps, and raising ocean levels. This might kick off a cycle of further warming.  Others, however, think that the forces that have caused Ice Ages in the past (i.e., Milankovitch cycles) will overshadow this human-induced warming, and we will once again move into a period of glaciation.

50 million years from now!Looking into the Future 50 million years

    However, the plates and continents aren't done moving yet!  If the current movements continue, more continental collisions are in the offing.  Africa will ram into Europe, raising a new giant mountain chain, making the Mediterranean Sea a thing of the past.  The southern California coast will slide northward, making Los Angeles the largest city in Alaska!  Perhaps a new Pangea will form as the continents move back together again.  North America may continue to rotate counterclockwise, swinging Pennsylvania farther north of the equator.  A new subduction zone may develop along the east coasts of both North and South America.  If this happens, say goodbye to the Atlantic Ocean!


Barnes, J. H.  and W. D. Sevon.  1996.  The Geological Story of Pennsylvania, 2nd ed.  Pennsylvania Geological Survey, 4th Series.  Educational Series 4.  44 pp.

These wonderful graphics, as well as other instructive 3-D animations and descriptive material, can be found at the PALEOMAP Project website:

Scotese, C.R., 2002,, (PALEOMAP website). 


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