Planet Earth/6j. Earth’s History Preserved in its Rocks: Stratigraphy and Geologic Time

The Discovery of Earth's Eons

Illustration made by Henry T De la Bech in 1832, titled "The Light of Science"

The portrait of Mary Fairfax Somerville, who published the influential book Physical Geography in 1848.

The horse drawn carriage bounced over the rocky road as Charlotte Murchison keenly observed the landscape they passed on her grand tour of the European countryside. Open at her side was a sketchbook, in which she roughly illustrated the craggy rocks and pillowing trees they passed on their journey east. She was born Charlotte Hugonin, the same year that James Hutton published his grand book on the Theory of the Earth in 1788. Her parents were wealthy and well educated and she grew up with an interest in natural history. At the age of 27, she caught the attention of a handsome soldier returning from the Peninsular War, Roderick Murchison, a military man who served in the British military during the Napoleonic Wars in Portugal and Spain. Well respected for his successful military career, he struggled to settle into the domestic life with his marriage to Charlotte. Charlotte acutely understood that the best way to keep him occupied was to engage him in a pursuit of science, in particular the study of minerals, rocks and fossils. Charlotte Murchison befriended many of the early natural historians of the day, who ventured out into the countryside to observe the natural landscape under a scientific appreciation. She met and befriended the early geologist Henry De la Beche and the paleontologist Mary Anning, two central figures in the study of rocks and fossils along the southern coast of England. Mary Anning had discovered bizarre fossils of aquatic dolphin-like reptiles called ichthyosaurs, and flying reptiles called pterosaurs from the ancient rocks along the coast. These fossils, as well as the terrible gigantic lizards illustrated by Mary Buckland and described by William Buckland at Oxford University (later named dinosaurs), revealed a deep and rich prehistoric history for the Earth. Such collection of rocks and bizarre fossils became a fashionable endeavor for the couple, and the venturing out into the countryside to collect and describe these rock layers in the sequences as they had been laid down over the eons represented a new endeavor that would keep her husband occupied. In 1816, a year after their marriage, she arranged a grand tour of France, the Swiss Alps, and Italy.

Painting my C. W. Peale the famed American naturalist in 1806 of the excavations of a Mastodon which was subsequently described by George Cuvier. It was the first animal that scientists hypothesized was extinct, and no longer living on Earth.

The trip was highly enjoyable for the couple and they continued such trips across Europe in the peaceful years that followed the defeat of Napoleon at the battle of Waterloo in 1815. They visited famous scientists of the day, including Mary Somerville, who had retired to Italy, and was currently working on her own description of the Earth in her classic book Physical Geography, a book that went beyond rocks and sediments, to include the geographic distribution of animals and plants on the surface of the Earth. They met with the famed French anatomist Georges Cuvier who had published a book in 1813 entitled Essay on the Theory of the Earth proposing that species of animals went extinct in the ancient past, and that the rock layers preserved extinct life forms punctuated by catastrophic events. There were major extinctions of ancient species that no longer lived on the Earth, such as an extinct elephant-like animal he named the Mastodon, the fossilized bones of which were sent to him from the United States, and compelled Thomas Jefferson to send Lewis and Clark westward to Oregon in a failed search for living examples of this extinct creature.

The Origin of the Geological Time Scale

Illustration of the underlaying geology from a figure illustrated by Mary Ann Mantell

These travels and visits were all captured by Charlotte in her sketchbooks, illustrating the many observations of fossils, rocks and the various layers of sedimentary rocks. She discovered similarities between rocks observed in England and Scotland, to those in the Italian and French Alps. They visited Germany, Italy, Prussia and even as far as Russia. In 1828, the couple invited a young aspiring geologist named Charles Lyell, whom they meet in Paris, and he too joined their horse drawn carriage traveling over the mountainous Alps. Charles Lyell was working on his important multivolume book entitled the Principles of Geology, which would be carried, read and later inspire Charles Darwin on his journey around the world. What Charlotte discovered in her travels was that there was an order and apparent sequence of fossils discovered within each strata or layer of sedimentary rocks, and each of these layers represented unique divisions of time. Her ideas made their way into the writings of her husband, but also mingled from the many scientists she interacted with on her travels through Europe.

Her husband would often adjourn to a tavern or inn to drink and there discuss the naming of these series of rock layers. The early Italian geologist Giovanni Arduino had divided sedimentary rock layers into four types in 1774, which he named Primary for rocks found in the central core of mountains, Secondary for tilted rocks next to mountains with unrefined and imperfect fossils, Tertiary for more horizontal rocks laid on top of secondary layers with fossils that were similar to those seen in the modern environment and Quaternary of loose sands and gravels that rest atop of tertiary rocks. However, these rock layers could be further subdivided, based on their lithological characteristics as well as their preserved fossils. The English geologist William Smith produced the first map of England showing the various exposures of the rock layers of the country in 1815, referred to as stratigraphy; the study of sedimentary rock layers. Smith’s adopted son, John Phillips published an early geological time scale in 1841, dividing the layers of rock based on the fossils they exhibited into Paleozoic (English spelling Palaeozoic) (ancient life), Mesozoic (middle life), and Cenozoic (modern life), which was, at that time called the Cainozoic. These three divisions were further divided.

The earliest geological map that shows the geology of the bedrock layers published by William Smith in 1815.

Geological Time Scale

For example, Charlotte’s husband, Rodrick, coined a period of time called Silurian recognized as a subdivision of the Paleozoic, for a series of rock layers in England, and wrote that the fossils found in these rock layers formed during a distant ancient time and represented species of extinct marine animals. Fossils when coupled with their order of superposition (in other words their relative position in the sequence of stacked rock layers and recognizing the youngest layers at the top and oldest layers at the bottom) was the major criteria for determining their ancient geological age. Other ages were proposed based on each layer’s unique assemblage of fossils; the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleocene, Eocene, Oligocene, Miocene, Pliocene and Pleistocene.

Layers of rock can be correlated across geographic regions based on their characteristics and fossil occurrences.

These collaborative efforts gave rise to a geological time scale that geologists use today, without any of the scientists knowing, at that time, the actual dates and lengths in terms of the number of years of each of these divisions of Earth’s history. In North America geological mapping became a major driving force with the westward expansion of railways that needed local sources of coal within these rock layers to run. Such investment into the understanding of the nature of sedimentary rock layers also decoupled the rock layers from their corresponding times. Hence these sedimentary rock layers could be named (as Formations) based on their lithological characteristics (a red sandstone versus a gray shale), while time was divided (as Periods) based on their fossil occurrences. The Mesozoic became known for its dinosaur fossils, while the Devonian was noted for its occurrence of early fossil fish. These divisions were temporal, a result of key extinction events in Earth’s history.

Earth's Photo Album

The geological time spiral, showing the major evolutionary events of life on Earth through its long history.

A good analogy to understanding the geological time scale is to view it as a photograph album of Earth’s history, similar to a personal photograph album of your own life. There are photographs taken of your birth, as a baby, others while you were a child learning to walk, some photographs of family members that have passed, a high school graduation photograph, a marriage, a new home, a car you once owned, each photograph lacking a precise date or time that it was taken, but you are able to assemble the order (in part because each page contains photographs taken at the same time in your life). Such a photograph album would serve as a chronology of your own life, even if you did not know the precise dates that were recorded. Such is the fossil record of life on Earth. It records the events, fragmentary for sure, but like a photograph album able to preserve enough for someone to understand those events, or at least interpret them. James Hutton viewed Earth’s history both as never ending and unchanging through its history, what Charlotte Murchison uncovered with her numerous colleagues in the early 1800s was that Earth’s history was long, but always changing, as life seemed to respond to environmental, climatic, and ecological changes as they were preserved as fossils chiseled and dug up from the countryside. Each fossil revealing a fragment to a story of Earth’s history. Major extinctions, great origins, and breakthroughs, such as when life conquered land, the ever-changing world, not only dictated by Earth’s surface processes, but by the biological events that altered those events, such as the advent of terrestrial plants and animals, of monstrous creatures and the strange and beautiful beasts that evolved, changed, conquered, and then perished from the Earth, only to be preserved as fossilized skeletons for eons in the subsurface of this planet.

The Geological Time Scale

Precambrian (the time of no macrofossils, the Primary Rocks)

Hadean Eon (4,543 million to 4,000 million years ago)

The Hadean was Earth's molten period early in its history.

The Hadean Eon is a long period of time lacking any representative rocks and fossils, as the Earth was likely molten during this early time. The existence of such a nearly unknown period of Earth’s early history is only inferred from radiometric ages of meteorites that extend the age of the solar system back 4,543 million years ago, but the oldest terrestrial rocks formed on Earth are dated only back as far as 4,000 million years ago. These ancient terrestrial rocks are individual grains of hard zircon crystals preserved in the most ancient metamorphic rocks that are found in the cratons of continents, like the Canadian Shield.

Archean Eon (4,000 Million to 2,500 Million years ago)

The Archean was a long period of Earth's history where life first appeared in the oceans.

The Archean Eon is the long period of time in which life arose as unicellular, single celled lifeforms, such as bacteria. Earth’s atmosphere and oceans lacked significant free oxygen atoms. All life on the planet was microbial living in an ocean were oxygen was rare (anoxic). Photosynthesizing lifeforms that utilized carbon dioxide from the atmosphere arose during this time, as cyanobacteria (blue-green algae). Fossils of these bacterial mats, called stromatolites, appear in the rocks deposited during this time, with more abundance of fossils seen near 2,500 million years ago. The eon ended with the Great Oxygen Event, where oxygen increased to such levels in the atmosphere to result in the first open combustion of flames on Earth’s surface, as well as the possible extinction of many types of microbial life.

Proterozoic Eon (2,500 million to 541 million years ago)

Stromatilite, layers of fossilized blue-green algae that can been found in rocks from the Proterozoic.

The Proterozoic Eon is the long period of the second half of the Precambrian, lasting nearly 2 billion years. It has been subdivided, but is a long period represented by mostly single celled life. During this long eon of time, the first eukaryotic celled animals and plants arose. It was a period in which Earth’s atmosphere was rich in free oxygen. The abundance of oxygen in the atmosphere lead to the dramatic cooling of the Earth multiple times, resulting in periods of the nearly complete freezing of the Earth’s oceans, a particularly episodic climatic event during the last 300 million years of the Eon. Earth’s oceans nearly froze and thawed multiple times during this long episode in Earth’s history, but life remained mostly unicellular during this major expanse of time. Despite an eon of time in which life was only represented by single cells, these unicellular organisms continued to adapt and change to ever changing environments, and slowly become more complex. In the final millions of years of this long episode of Earth’s history, the first multicellular organisms arose, a period called the Ediacaran (635-541 Million years ago) which preserves soft-tissue impressions of animal-like and plant-like fossils.

Phanerozoic Eon (the time of abundant macrofossils, the Secondary, Tertiary and Quaternary Rocks)

Paleozoic Era (The Time of Ancient Life)

Cambrian Period (541 million to 485.4 million years ago)

Trilobites are common fossils in the Cambrian Period, rocks of this age are common in Western Utah, this trilobite (Olenoides superbus) is from the Marjum Formation in the House Range of Utah.

The Cambrian Period is the first period of the Paleozoic Era, and Phanerozoic Eon, represented by the earliest multicellular lifeforms that are preserved with hard skeletons. One of the commonly occurring fossils in marine rock layers of this age are trilobites, small insect-like arthropods that scurried across the ocean floor. Many unusual and strange marine fossils are known, including early fossilized shelled organisms and marine sponges. An early swimming group of animals called the chordates lived during this time, and would later give rise to fish. Green and brown algae, examples of multicellular plant-life appear at this time in marine and freshwater environments, and the first major carbonate reefs were deposited in shallow seas that covered western Utah, laying down the rock layers in Cache Valley and along the Wasatch Range.

Ordovician Period (485.4 million to 419.2 million years ago)

Horn corals called Rugosa, are common in Ordovician aged rock layers along the Wasatch Mountain Range in Utah.

Rocks from the Ordovician Period were laid down above the older Cambrian rocks, and are known for their abundance of rugosa horn-corals, and graptolites, as well as some of the earliest jawless fish. Rocks from the Ordovician Period are preserved in the topographically high Wasatch Mountain Range, which at one time represented a shallow marine carbonate reef during this period of Earth’s history.

Silurian Period (443.8 million to 419.2 million years ago)

The Silurian Period was a time of the first major diversification of life on land, with the first terrestrial vascular plants and insects appearing, while fish continued to diversify in the oceans and rivers.

Devonian Period (419.2 million to 358.9 million years ago)

Reconstruction of the early forests of the Devonian Period.

The Devonian Period is often called the Age of Fish, as the seas and rivers hosted a wide diversity of fossil sharks, placoderms (armored fish), and fast-moving jawed fish. Toward the end of the period the first terrestrial vertebrates (animals with back bones) appeared like the transitional fossil Tiktaalik.

Tiktaalik an early tetrapod, one of the first vertebrates to "walk" on land.

Mississippian Period (358.9 million to 323.2 million years ago)

Reconstruction of crinoids that lived in the oceans during the Mississippian Period.

The Mississippian Period is the first part of the Carboniferous, a time of dense forests and swamps, rich insect life, and early reptiles and amphibians. Marine life was particularly diverse with abundant brachiopods (shelled organisms) and echinoderms like crinoids (ancient sea lilies).

Pennsylvanian Period (323.2 million to 298.9 million years ago)

The Pennsylvanian Period saw the first divisions between egg-laying reptiles, as vertebrate animals were able to lay eggs outside of the water. In coastal Utah, major deserts and a warmer climate appeared, while dense forests continued in the Eastern Utah States, preserved as major coal deposits. A mountain range called the Ancestral Rocky Mountains and Antler Orogeny appeared in the western Utah States. The Pennsylvanian Period is the second half of the Carboniferous.

Permian Period (298.9 million to 251.9 million years ago)

Edaphosaurus a fossil from the Permian Period found in Texas and Southeastern Utah.

The Permian Period was a time of warming climates, more oxidizing red beds, and the rise of early mammals, which still closely resembled reptiles, but others may have had fur. It was also a period of time during the formation of the Super Continent Pangea, where all the continents had come together on one side of the Earth. This strange configuration resulted in a climate that oscillated from wet to dry, leaving thick red beds of sandstone and mudstone. Early English geologist confused red sandstones from the Devonian Period (called the Old Red Sandstone) with red sandstones from the Permian Period (called the New Red Sandstone), noting that fossils were different from these similar looking layers of sedimentary rocks. The end of Permian Period saw the largest mass extinction on Earth, resulting from the major release of carbon dioxide and other volcanic gasses from the eruption of the Siberian Traps. The intense hot climate and de-oxygenation of the quickly acidified oceans, resulted in a major turnover of life on the planet. Major groups of animals and plants vanished from the fossil record after the Permian, including trilobites, and most brachiopods.

Mesozoic Era (The Time of Middle Life, or the Age of Dinosaurs)

Triassic Period (251.9 million to 201.3 million years ago)

The late Triassic Period was the first time dinosaurs appear on Earth.

The Triassic Period is the first of the three periods of the Mesozoic Era, when dinosaurs first arose to dominate the Earth. Early flying pterosaurs appear for the first time, as well as fast moving bipedal ancestral dinosaur-like reptiles. Animals and plants slowly recovered during this span of time, but the climate oscillated between extremes, as the Super Continent begin to break apart with the opening of the Atlantic Ocean. The oldest dates from basaltic sea floor igneous rocks are known from this time, meaning nearly all igneous rocks from the ocean floor are younger than the Paleozoic. The end Triassic extinctions resulted in a new era of a stable humid warm climate, and rise of gigantic dinosaurs.

Jurassic Period (201.3 million to 145.0 million years ago)

Allosaurus is a common dinosaur found in the late Jurassic Period, this dinosaur is found in the Morrison Formation from Eastern Utah.

The Jurassic Period is iconic for its dinosaurs, like Apatosaurus that roamed across eastern Utah, but also large predatory dinosaurs like Allosaurus. The Jurassic Period was also the time that mammals became tiny, and birds first appeared with flight feathers. Conifer forests, ferns and horse tails covered the land, with major rivers flowing over a warmer Earth.

Cretaceous Period (145.0 million to 66.0 million years ago)

The Cretaceous Period was the time of Triceratops.

The Cretaceous Period marks a major diversification in plant life, with the origin of flowering plants, fruit and nuts, with the major group of plants called angiosperms first appearing in the fossil record. These plants use pollinating insects and other animals to carry pollen between plants, rather than wind. To help with seed dispersal these plants also produced endosperm, as fleshy tasty fruits that animals feed on and carried seeds with them, and to be fertilized once digested with their excrement. The major plant diversification resulted in mammals specialized in eating fruit and nuts, as well as the continued diversification of large dinosaurs, including Triceratops and Tyrannosaurus. Modern teleost fish, ancestors of the gold fish and perch first appear. The end of the Cretaceous witnessed a major extinction, with evidence that a large meteorite struck the Earth near the Yucatan Peninsula of Mexico, resulting in the extinction of the large non-avian dinosaurs. This extinction (sometimes called the K-T boundary, as K is used to abbreviate Cretaceous, and T to mark the beginning of the Tertiary, or K-Pg boundary) resulted in a new era, that brought a rapid diversification of mammals.

Cenozoic Era (The Time of Modern Life)

The Cenozoic Era has traditionally been divided into the Tertiary and Quaternary, but new discussions have divided the Tertiary into two periods called the Paleogene and Neogene, while retaining the term Quaternary for rocks and sediment 2.58 million years or younger. Most geologists divide this time using the term Epoch (pronounced Epic in American English, and EE-Pock in England), first proposed by Charles Lyell.

Paleocene Epoch (66.0 million to 56.0 million years ago)

The Paleocene Epoch was the first ten million years after the extinction of the dinosaurs. The time presents a world still recovering from the latest mass extinction, with a rapid diversification of terrestrial mammals and birds. Many strange and archaic mammals are known during this time, including the ancestors that would become primates. The Paleocene Epoch ended with a major global warming event called the Paleocene-Eocene Thermal Maximum, that was brought about by the opening of the Arctic Ocean through mid-ocean rifting and the massive release of methane hydrate from the deep ocean.

Eocene Epoch (56.0 million to 33.9 million years ago)

Eohippus a common tiny horse ancestor, found in North America during the Eocene Epoch.

The Eocene Epoch was a long epoch of warm climates, when great forests grew across the Earth, filled with arboreal animals, such as early primates that leaped between the high canopy trees. Tiny horses running through the dense bush, chased by hooved predators. An ice-free world of lush crocodile filled lakes, and well drained rivers, with isolated continents of South America, Australia and Africa, while North America, Europe and Asia shared a land bridge in the temperate high arctic, allowing wandering large mammals to roam between the northern continents, leaving the southern continents isolated, with endemic faunas of strange beasts, like the earliest elephants, and bizarre horned mammals. The end Eocene saw a major cooling of the Earth, as South America and Australia broke away from Antarctica, allowing the return of great ice sheets in the southern pole.

Oligocene Epoch (33.9 million to 23.03 million years ago)

Hoplophoneous an early sabertooth cat from the late Eocene/early Oligocene White River Formation in North America.

The Oligocene Epoch witnessed a drier, and slightly cooler time period, as the great forests changed into open woodlands, with expanding grasslands, and running hooved mammals, like early deer, and the first sabretooth cats that chased them. The ancestor of wolves and dogs appear, while rabbits and rodents were more common. Primates lost their tails, with a group specific to Africa that began to dominate in the more open habitat of that large continent. At the end of the Oligocene, horses nearly went extinct, and the landscape became drier with vast tree-less grasslands appearing.

Miocene Epoch (23.03 million to 5.333 million years ago)

The Miocene Epoch is the time of the apes, as a small group of primates in Africa moved into these more open grasslands and hunted for food as omnivores. Large elephant-like creatures journeyed out of Africa, and across the northern continents, while herds of antelope, even toed ungulates raced across savannahs. The climate was warm, but rise of the Himalayan Mountains of the Tibet Plateau rose to great heights, with the continued collision of India into Asia, and the Mediterranean Sea dried up at the end of the Miocene Epoch during the Messinian Event.

Pliocene Epoch (5.33 million to 2.58 million years ago)

Australopithecus skeletons from the Pliocene Epoch of Africa.

The Pliocene Epoch was the age of Australopithecus, the early hominid, and member of your family. A small tail-less bipedal creature who roamed across Africa, and climbed trees when confronted with a predator. They moved in groups, and may have used tools. The climate continued its cooling, resulting in ice sheets across Europe, Asia and North America that would continue into the Great Ice Ages of the last Epoch.

Pleistocene Epoch (2.58 million to 0.0117 million years ago)

Mastodon skeletons from rock layers deposited during the Pleistocene Epoch.

The Pleistocene Epoch is often placed within the last series of rocks, the loose unconsolidated soil and dirt not yet lithified into rocks, the Quaternary, which also includes the modern age. The Pleistocene Epoch is the great period of Ice Ages, in which ice sheets covered North America, Europe and Asia, lowering sea level. Wooly rhinos and mammoths roamed these frozen steppes. Early humans ventured out of Africa for the first time tracking the warming climates first, but later learning to hunt these large beasts with spears and tools. The epoch was characterized by cyclic ice ages, punctuated by interglacial warmer times, it was during the last of these great interglacial warm periods, when humans descended into North America, South America, Australia and beyond that marks the end of the Pleistocene Epoch, because this incursion of humans brought a major extinction of the great megafauna, the gigantic beasts that they hunted.

Holocene Epoch (0.0117 million years ago to today)

The Holocene is a tiny sliver of time in which we have written words, but characterized by the end of the ice ages, and a warm period that resulted in a massive increase in a single species that would dominate Earth, Homo sapiens, humans. The Earth underwent a major change as ice and forests were carved into asphalt and concrete, the planet warmed with increased carbon dioxide and grasslands were altered to massive agricultural regions of crops planted to feed a single species. The Holocene Epoch is the time we live within, although some argue that even this tiny sliver of time, only 11,700 years long should be even further subdivided.

When Charlotte Murchison bounced along in a horse drawn carriage two hundred years ago, she may have understood only a tiny aspect of the great vastness of time preserved in the rock layers she sketched, like the sense of emptiness when one views far away stars in the night sky. The Earth and its ages seem immense, an endless journey of time, filled not only with the small daily processes of erosion and transport of individual sediments, but of cataclysmic events that altered whole sets of organisms and ecologies of the planet. Great periods on Earth when it resembled a frozen white planet, or periods when it was a red yellow planet of vast sand deserts, or times it was a green planet of prodigious forests encompassed by blue oceans, oceans ever changing in their composition of life. This is Earth’s story, vast, dynamic, and without a real end. It is also the story of life on Earth. Life that one part of Earth that makes this planet so unique, maybe the only planet in the universe with this strange occurrence of intelligence, and conscious thought. How did this come to be?

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i. Earth’s Surface Processes: Sedimentary Rocks and Depositional Environments.	j. Earth’s History Preserved in its Rocks: Stratigraphy and Geologic Time.	a. How Rare is Life in the Universe?