Geological Time Scale

The geological time scale explained, eon by eon

What follows is the whole scale in order, oldest first, with every eon, era and period given its own section and every Cenozoic epoch as well. Each heading is live. Click it and the chart above jumps to that interval, selects it and opens its detail panel, so you can read the description here and look at the boundaries, events and GSSP at the same time.

Two things are worth fixing in your head before you start. The ranks nest strictly: an eon contains eras, an era contains periods, a period contains epochs, and an epoch contains ages. And the units get shorter as you move towards the present, not because time sped up but because the record improves. The Norian lasted 21.6 million years. The Greenlandian lasted 3.5 thousand years. They sit at the same rank on the same chart.

The second thing is proportion. Everything with a visible fossil record, the whole Phanerozoic, is 11.8 percent of Earth history. The other 88 percent, the Hadean, Archean and Proterozoic together, is usually squashed into a strip at the bottom of a printed chart. On this tool you can open it out and see how much room it really takes.

• Hadean
• Archean
• Proterozoic
• Phanerozoic
• Cambrian
• Ordovician
• Silurian
• Devonian
• Carboniferous
• Permian
• Triassic
• Jurassic
• Cretaceous
• Paleogene
• Neogene
• Quaternary

Hadean Eon, 4567 to 4031 Ma

The Hadean is the only eon with almost no rocks in it. It runs from the moment the Earth finished accreting, taken as 4567 Ma from the age of the oldest calcium and aluminium rich inclusions in meteorites, down to 4031 Ma where the oldest reliably dated crust begins. Everything in between has to be reconstructed from meteorites, from the Moon, and from a scatter of mineral grains that outlived the rocks they formed in.

The defining event is the giant impact at roughly 4.51 Ga that produced the Moon. It melted a large fraction of the planet, and the magma ocean that followed let iron sink to the centre, separating core from mantle within about the first 30 million years. That single episode set the mass of the core, the composition of the mantle, the tilt of the axis and the initial spin. The day was about six hours long and the Moon was close enough that tides were hundreds of times larger than today.

The surprise in the Hadean is how quickly it cooled. Detrital zircons from the Jack Hills of Western Australia, the oldest at 4.404 Ga, carry oxygen isotope ratios that only make sense if they crystallised from magma that had interacted with liquid water at low temperature. That pushes oceans back to within 150 million years of planetary formation, which is a very different picture from the permanent lava world the name Hadean suggests. There is no GSSP here and there never will be. The eon remains informal.

Archean Eon, 4031 to 2500 Ma

The Archean is where the rock record starts. Its base is set numerically at 4031 Ma to match the Acasta Gneiss of the Northwest Territories, and it runs to 2500 Ma. Mantle temperatures were 200 to 300 degrees Celsius higher than today, which shows up directly in the lavas: komatiites erupted at around 1600 degrees Celsius and flowed like water, and no komatiite younger than the Proterozoic exists because the mantle has never been that hot since.

Continental crust appears in the form of tonalite, trondhjemite and granodiorite suites wrapped around greenstone belts of pillow basalt and chert. Whether plate tectonics as we know it was operating is still argued. What is clear is that by the end of the eon there were rigid cratons stable enough to survive to the present, and those Archean cores sit inside almost every continent today.

Life was already here. Carbon isotope values near minus 25 per mil in graphite from Isua in Greenland point to biological carbon fixation by about 3.8 Ga, and the Dresser Formation of the Pilbara preserves layered stromatolite mounds at 3.48 Ga that are hard to explain without microbial mats. The atmosphere held no free oxygen, the Sun was only about 75 to 80 percent as bright as it is now, and the greenhouse that kept the oceans liquid was probably carbon dioxide with a contribution from methane.

Eoarchean Era, 4031 to 3600 Ma

The Eoarchean holds the oldest surviving crust anywhere: the Acasta Gneiss at 4.03 Ga and the Nuvvuagittuq greenstone belt in Quebec, which yields at least 4.16 Ga and possibly more. The Isua supracrustal belt in Greenland, around 3.8 Ga, is the oldest sequence that still looks like ordinary sedimentary and volcanic rock rather than a heavily reworked gneiss, and it is the source of the earliest carbon isotope evidence for life. Almost all of it has been metamorphosed at amphibolite grade or higher, so every interpretation is an argument about what survived.

Paleoarchean Era, 3600 to 3200 Ma

The Paleoarchean is the first era with reasonably well preserved rocks in quantity, mostly in two places: the Pilbara Craton of Western Australia and the Barberton greenstone belt of South Africa. Both preserve pillow lavas, cherts, komatiites and the oldest widely accepted microfossils and stromatolites. Barberton also carries spherule beds from large impacts, layers of once molten rock droplets that rained out of the atmosphere after collisions far bigger than anything in the later record.

Mesoarchean Era, 3200 to 2800 Ma

By the Mesoarchean the cratons were large enough to carry their own sedimentary cover, which is why the era gives us the first proper sedimentary basins rather than just greenstone belts. The Pongola Supergroup of South Africa contains glacial diamictites at about 2.9 Ga, the oldest evidence of ice on Earth, which is remarkable given a fainter Sun and no oxygen. Whatever caused it, the planet recovered and stayed warm for another 500 million years.

Neoarchean Era, 2800 to 2500 Ma

The Neoarchean is the great crust building era. A large fraction of all continental crust that has ever existed was extracted from the mantle between 2.8 and 2.5 Ga, and the largest banded iron formations on the planet, including the Hamersley and Transvaal successions, were laid down in its final 100 million years. Those iron formations require dissolved ferrous iron in the deep ocean and an oxidant at the surface, which is the chemical signature of cyanobacteria producing oxygen faster than the ocean could stay reduced. Trace metal and sulfur isotope data record short lived oxygen whiffs from about 2.7 Ga, well before the atmosphere itself changed.

Proterozoic Eon, 2500 to 538.8 Ma

The Proterozoic covers 1961 million years, which is close to four times the length of the entire Phanerozoic, and it contains the two largest changes in Earth surface chemistry ever recorded. It opens with the arrival of free oxygen and closes with animals. In between it holds a supercontinent cycle, a billion years of apparent stability, and two episodes where ice reached the equator.

All ten Proterozoic periods are defined numerically rather than by a golden spike, with one exception at the very top. That is not laziness. Fossils useful for global correlation barely exist, so the subcommittees picked round numbers and moved on. Click any Proterozoic period on the chart above and the panel will say Global Standard Stratigraphic Age rather than showing you a section.

Paleoproterozoic Era, 2500 to 1600 Ma

The Paleoproterozoic is the most eventful era in the Precambrian. It contains the Great Oxidation, the first global glaciations, the two largest impact structures on Earth, and the assembly of the first well documented supercontinent. If you only look at one part of the Precambrian in detail, look at this one.

Siderian Period, 2500 to 2300 Ma

The Siderian is named for iron, and it earns it. This is the peak of banded iron formation deposition, the alternating layers of iron oxide and chert that supply most of the world iron ore. The mechanism needs a stratified ocean with dissolved ferrous iron below and an oxidising surface above, so the Siderian is really the last stretch of the old anoxic world running at full capacity while oxygen accumulates at the top of the water column.

Rhyacian Period, 2300 to 2050 Ma

The Rhyacian carries the Great Oxidation Event and the Huronian glaciations, and the two are almost certainly connected. Once oxygen exceeded roughly 0.001 percent of the present atmospheric level, the mass independent fractionation of sulfur isotopes that had persisted for two billion years switched off permanently, which is the sharpest chemical boundary in the whole geological record. Oxygen also destroys atmospheric methane, and methane was doing a lot of the greenhouse work, so removing it plunged the planet into at least three glacial episodes that may have reached the tropics.

The Rhyacian also hosts the Bushveld Complex at 2.06 Ga, a layered mafic intrusion covering 65 thousand square kilometres that holds around three quarters of the world platinum reserves. It is the largest body of its kind anywhere and it formed in under a million years.

Orosirian Period, 2050 to 1800 Ma

The Orosirian is the era of mountains, with orogenic belts of this age on essentially every craton as the fragments assembled. It also contains both giant impacts: Vredefort in South Africa at 2.023 Ga, originally 250 to 300 kilometres across, and Sudbury in Ontario at 1.849 Ga, whose melt sheet hosts one of the richest nickel and copper districts on the planet.

Sitting across the Rhyacian and Orosirian boundary is the Lomagundi excursion, the largest and longest positive carbon isotope anomaly in Earth history, with carbonate values reaching plus 10 per mil and holding for roughly 130 million years. That means organic carbon was being buried on an extraordinary scale, which is the other side of the oxygen story: every buried carbon atom leaves an oxygen molecule behind.

Statherian Period, 1800 to 1600 Ma

The Statherian is when the pieces locked together into the supercontinent variously called Columbia or Nuna, and cratons that had been drifting independently for a billion years became a single mass. Stable platforms mean thick undeformed cover sequences, and those sequences give us the first reasonably confident eukaryote fossils, acritarchs with complex ornamented walls that no prokaryote is known to make. The coiled fossil Grypania, at up to a centimetre long, appears at about 1.87 Ga in Michigan.

Mesoproterozoic Era, 1600 to 1000 Ma

The Mesoproterozoic has a nickname that has stuck: the boring billion. Carbon isotopes are flat, oxygen sits at a low intermediate level, sea level barely moves, and biological innovation appears to stall. Whether that reflects genuine stasis or just a thin and poorly dated record is an active argument, and evidence is accumulating that it was less boring than the name implies. What is not in doubt is that the era ends with the biggest mountain building event of the Precambrian.

Calymmian Period, 1600 to 1400 Ma

The Calymmian is named for the cover that spread over the newly stable cratons. Its rocks are dominated by platform sequences, sandstones and carbonates that were never strongly deformed, which is why they are geologically dull and stratigraphically useful at the same time. The oldest good evidence for complex multicellular life comes from close to the base of this period, in the Indian phosphorites at about 1.6 Ga, where Rafatazmia and Ramathallus preserve cell walls and pit connections that look red algal.

Ectasian Period, 1400 to 1200 Ma

The Ectasian, meaning extension, continues the same story of spreading cover with a slow pulse of rifting. Its most quoted fossil is Bangiomorpha pubescens from Arctic Canada at about 1.05 Ga, a filamentous red alga that shows differentiated cells, a holdfast and, most importantly, spores. It is the oldest organism for which sexual reproduction can be demonstrated from the fossil itself.

Stenian Period, 1200 to 1000 Ma

The Stenian is defined by the narrow high grade metamorphic belts that formed as Rodinia came together, above all the Grenville orogeny. Grenville age rocks run from Mexico up through eastern Canada, across Scandinavia and into Scotland, and the mountains they represent were Himalayan in scale. The erosion of a belt that size delivers nutrients to the ocean at a rate that shows up in later chemistry, which is one reason the Stenian and the Tonian that follows it look so different.

Neoproterozoic Era, 1000 to 538.8 Ma

The Neoproterozoic is where the modern world starts to be recognisable. Rodinia comes apart, oxygen rises for the second time, ice covers the planet twice, and by the end there are large soft bodied organisms lying on the sea floor. It is also the only Precambrian era whose top is defined by a real golden spike.

Tonian Period, 1000 to 720 Ma

The Tonian opens with Rodinia intact and closes with it splitting along rifts that eventually became ocean basins. Rifting matters climatically because it creates long stretches of new coastline in the tropics, and tropical silicate weathering draws carbon dioxide out of the atmosphere. Several models put the trigger for what happens next right here. Biologically the Tonian gives us the first clear heterotrophic eukaryotes, including vase shaped microfossils that were almost certainly amoebae eating other cells.

Cryogenian Period, 720 to 635 Ma

The Cryogenian holds the two snowball Earth glaciations, and they are not subtle. The Sturtian ran from about 717 to 661 Ma, which is 56 million years of near global ice, and the Marinoan from about 639 to 635 Ma. Glacial deposits of both ages sit on continents that palaeomagnetic data place at the equator at the time, which is the core of the snowball argument.

The way out is as interesting as the way in. With the ocean sealed and weathering shut down, volcanic carbon dioxide accumulated until the greenhouse overwhelmed the ice, and the melt happened fast. The cap carbonates that sit directly on Marinoan glacial rock all over the world record that transition, a metre or two of pale dolostone deposited in perhaps a few thousand years on top of tillite.

Ediacaran Period, 635 to 538.8 Ma

The Ediacaran is the first period since the Archean to be defined by a section rather than a number, and the section is at Enorama Creek in the Flinders Ranges of South Australia, where the base of the Nuccaleena cap carbonate sits on Marinoan glacial deposits. That is why the base is 635 Ma and not a round 650.

The period is named for its biota: soft bodied, often centimetres to over a metre in size, and mostly impossible to place in any living group. Rangeomorphs such as Charnia had a fractal branching body plan with no mouth, no gut and no obvious way of moving. Dickinsonia left feeding traces, so something was eating. By the last few million years there are burrows, calcified tubes like Cloudina, and the first evidence of predation in the form of drill holes. The Gaskiers glaciation at about 580 Ma was brief, roughly 340 thousand years, and the largest Ediacaran fronds appear in Newfoundland immediately above it.

Phanerozoic Eon, 538.8 to 0 Ma

The Phanerozoic means visible life, and its base at 538.8 Ma is defined at Fortune Head in Newfoundland on the first appearance of the burrow Treptichnus pedum. It covers 539 million years, 11.8 percent of Earth history, and it contains essentially everything most people picture when they think about the geological past. It is divided into three eras whose boundaries are both mass extinctions, which tells you how the divisions were arrived at in the nineteenth century: the fossils changed, so the names changed.

Paleozoic Era, 538.8 to 251.902 Ma

The Paleozoic runs 287 million years and it is the era in which life colonised everything. It starts with animals confined to the sea and ends with forests, insects, reptiles and a single supercontinent. It also contains three of the five largest mass extinctions, including the worst of them all at its top.

Cambrian Period, 538.8 to 486.85 Ma

The Cambrian is famous for one thing, and the fame is deserved. Within roughly 25 million years of the base, almost every animal phylum with a hard skeleton shows up in the record, and it does so more or less at once. The Chengjiang biota of China and the Burgess Shale of British Columbia preserve soft tissue as well, which is how we know the diversity was real rather than an artefact of who happened to mineralise first.

What actually changed is contested but the ingredients are agreed: oxygen crossed a threshold that allowed active predators with circulatory systems, and once predation existed, skeletons, eyes, burrowing and fast movement all became worth the energy. The sediment record changes at the same time. Microbial mats that had covered the sea floor for a billion years were destroyed by burrowing animals, which is called the substrate revolution and shows up as a permanent change in trace fossil style.

The period is divided into four epochs: Terreneuvian, Series 2, Miaolingian and Furongian. Two of those still carry provisional names because their defining fossils have not been agreed, and four of the ten Cambrian stages have no GSSP at all. Tectonically Gondwana was assembling, sea level was rising through most of the period, and climate was a greenhouse with no polar ice.

Ordovician Period, 486.85 to 443.1 Ma

The Ordovician sees marine diversity roughly triple in what is called the Great Ordovician Biodiversification Event. This is a different kind of change from the Cambrian explosion: no new body plans, but an enormous increase in the number of families and genera and in ecological complexity. Brachiopods, bryozoans, crinoids, corals and cephalopods built the Paleozoic evolutionary fauna, and the first true reefs since the Archean stromatolites appeared.

Land was invaded during this period too. Cryptospores from Argentina and Oman at around 470 Ma record liverwort grade plants, and once plants were on land they began weathering rock and making soil, which slowly changed atmospheric chemistry. Sea level reached the highest values of the entire Phanerozoic, with epicontinental seas flooding continental interiors.

Then it ends abruptly. Gondwana drifted over the South Pole, an ice sheet centred on what is now the Sahara grew, sea level dropped roughly 100 metres, and the epicontinental seas drained. The Hirnantian extinction that followed killed about 85 percent of marine species in two pulses, one from the cooling and habitat loss and one from the warming and anoxia when the ice melted.

Silurian Period, 443.1 to 419.62 Ma

The Silurian is short, 23.5 million years, and it is a recovery period. Reefs rebuilt, this time with tabulate and rugose corals plus stromatoporoid sponges, and the seas of Gotland and the Welsh Borderland preserve them beautifully. Graptolites, colonial planktonic hemichordates, evolved fast and drifted globally, which makes them the best correlation tool of the period and explains why so many Silurian GSSPs are defined on graptolite first appearances.

The big step is vascular plants. Cooksonia, a few centimetres tall with a simple branching stem and sporangia at the tips, is the first plant with proper conducting tissue, and conducting tissue is what lets a plant stand upright and grow away from a permanently wet surface. Jawed fish also radiate here, and the first fully terrestrial arthropods, including millipedes and early arachnids, appear.

The Caledonian orogeny was closing the Iapetus Ocean, welding Laurentia to Baltica and Avalonia and raising a mountain belt whose eroded roots are now spread across Scotland, Scandinavia, Greenland and the northern Appalachians. The molasse shed off those mountains became the Old Red Sandstone. The Silurian is divided into four series, Llandovery, Wenlock, Ludlow and Pridoli, and the Pridoli is the only series on the whole chart with no stages inside it.

Devonian Period, 419.62 to 358.86 Ma

The Devonian is called the Age of Fishes and it is a fair label. Placoderms, some of them six metres long with shearing bony plates instead of teeth, shared the water with early sharks, lobe finned fish and the first ray finned fish. Ammonoids appeared and began the run that would make them the most useful Mesozoic index fossil.

On land the change was bigger. Archaeopteris, the first tree with a modern woody trunk and a proper root system, built the first forests at about 385 Ma, and roots that go metres deep do something no previous plant could: they physically break rock and pump acid into it. Silicate weathering rates jumped, carbon dioxide fell by roughly an order of magnitude across the period, and the first real soils formed. Tetrapods walked out of the water in the same interval, with trackways in Poland at 393 Ma and body fossils such as Ichthyostega and Acanthostega later.

The cost came at the end. The Kellwasser event at the Frasnian and Famennian boundary destroyed the reef ecosystems that had taken 100 million years to build, and the Hangenberg event at the very top cut jawed vertebrate diversity by around 96 percent, a bottleneck that shaped every fish and tetrapod lineage that followed. Nutrient runoff from the new forests driving ocean anoxia is the leading explanation for both.

Carboniferous Period, 358.86 to 298.9 Ma

The Carboniferous is the coal period, and the coal is the point. Equatorial swamps filled with lycopsid trees up to 40 metres tall, growing on waterlogged ground where dead wood did not fully decay, buried organic carbon at a rate the planet has never matched since. That burial pushed atmospheric oxygen towards 30 percent or more and drew carbon dioxide down to values close to the modern range.

High oxygen has visible consequences. Insects breathe by diffusion through tracheae, so their maximum size scales with oxygen availability, and the Carboniferous produced Meganeura, a griffinfly with a wingspan approaching 70 centimetres, along with two metre millipedes. Amniotes appeared too: Hylonomus, found inside hollow fossil tree stumps at Joggins in Nova Scotia at 318 Ma, laid a shelled egg and so cut the last tie to standing water.

With carbon dioxide low and Gondwana over the pole, ice sheets grew and the Late Paleozoic Ice Age ran for over 100 million years. Its orbital cycles show up as cyclothems, the repeating coal, shale and limestone packages that make Carboniferous stratigraphy so distinctive. The period is split into two subsystems, the Mississippian and the Pennsylvanian, which is a compromise between North American usage, where they are separate periods, and European usage, where they are not.

Permian Period, 298.9 to 251.902 Ma

The Permian is the period of one continent. Pangaea was fully assembled, running pole to pole, and a landmass that size generates a monsoonal interior with extreme seasonality and vast dry regions. That is why the Permian record is full of red beds, aeolian sandstones and evaporites, and why the coal swamps of the Carboniferous disappear.

Life on land diversified into the first properly modern terrestrial ecosystems, with synapsids as the dominant large tetrapods, including the sail backed Dimetrodon early on and the therapsids, our own ancestors, later. In the sea, fusulinid foraminifera grew to the size of a coin and reefs were built by sponges and algae rather than corals.

Then it ends with the worst event in the history of animal life. The Capitanian crisis at about 259 Ma, linked to the Emeishan flood basalts, was already a serious extinction. The main event at 251.902 Ma removed roughly 81 percent of marine species in under 60 thousand years. The Siberian Traps erupted through coal and evaporite deposits, releasing carbon dioxide, methane and halocarbons, and the ocean warmed, lost oxygen and acidified at the same time. Recovery took five million years, and the world that came back was run by different animals.

Mesozoic Era, 251.902 to 66 Ma

The Mesozoic is bracketed by the two most severe extinctions in the Phanerozoic and is best understood as the era in which Pangaea came apart. Continental breakup created new ocean basins, flooded shelves, raised sea level and kept carbon dioxide high, which is why the whole era is a greenhouse with no permanent polar ice.

Triassic Period, 251.902 to 201.4 Ma

The Triassic begins in an emptied world. The Early Triassic is a genuinely strange interval: reefs are absent for several million years, coal is absent worldwide for around 20 million years, and equatorial sea surface temperatures reached values in the high thirties Celsius that appear to have excluded large animals from the tropics altogether.

Recovery, when it came, was fast and creative. Archosaurs split into the crocodile line and the bird line within a few million years, and by 231 Ma Eoraptor and Herrerasaurus in Argentina are unambiguous dinosaurs. Mammaliaforms such as Morganucodon appear near the top of the period, shrew sized, with the fully mammalian jaw joint and an enlarged brain. In the sea, ichthyosaurs and nothosaurs took over the niches left by extinct Paleozoic predators.

Two disruptions punctuate it. The Carnian Pluvial Episode at around 233 Ma, coinciding with the Wrangellia flood basalts, broke a long arid regime with roughly a million years of heavy rainfall and coincides with the first dinosaur diversification. Then at 201.4 Ma the Central Atlantic Magmatic Province erupted across ten million square kilometres as Pangaea rifted, roughly doubling atmospheric carbon dioxide, acidifying the oceans and ending conodonts along with most large amphibians. The niches that cleared were filled by dinosaurs.

Jurassic Period, 201.4 to 143.1 Ma

The Jurassic is the period of breakup made visible. The Central Atlantic opened, Gondwana began to separate from Laurasia, and the Tethys Ocean widened westward. Flooded continental shelves under a warm greenhouse produced the thick marine mudstones and limestones that define the period in Europe and that later became major petroleum source rocks.

Ammonites are the reason Jurassic stratigraphy is so finely resolved. They evolved fast, floated widely and are abundant, which supports a zonal scheme of around 60 ammonite zones with an average duration under a million years each. Most Jurassic GSSPs are defined on ammonite first appearances for exactly this reason.

On land the sauropods reached their maximum size and the Morrison Formation of the western United States preserves the classic assemblage. Archaeopteryx, from the Solnhofen limestone of Bavaria at 150.9 Ma, carries asymmetric flight feathers alongside teeth and a long bony tail. The Toarcian Oceanic Anoxic Event at about 183 Ma, driven by the Karoo and Ferrar magmatism, warmed the sea by roughly 7 degrees Celsius and laid down black shales across Europe.

Cretaceous Period, 143.1 to 66 Ma

The Cretaceous is the longest period of the Phanerozoic at 77.1 million years, and it needs the length because so much changes inside it. Sea level reached its Phanerozoic maximum, roughly 200 to 250 metres above today, and shallow seas split North America along the Western Interior Seaway and flooded much of Europe. The chalk that gives the period its name is the accumulated coccolith debris of that ocean, and in places it is hundreds of metres thick.

Flowering plants are the other headline. They are present but rare in the Early Cretaceous, and by the end of the period they are over 70 percent of the land flora. They coevolved with bees, beetles and other pollinators, and the whole modern terrestrial ecosystem, with social insects, modern birds, lizards, snakes and early placental mammals, assembled alongside them in what is called the Cretaceous Terrestrial Revolution.

Two oddities are worth knowing. The Cretaceous Normal Superchron ran from 125.93 to 83.64 Ma with no full magnetic reversal, which strips 42 million years of rock of any magnetostratigraphic resolution. And the two big oceanic anoxic events, OAE1a at about 120 Ma and OAE2 at 94 Ma, buried carbon on a scale that laid down a large share of the world petroleum source rock while removing roughly 27 percent of marine invertebrate genera.

The end is the best dated catastrophe in the record. Deccan volcanism was already underway when a bolide about ten kilometres across struck Yucatan at 66.05 Ma, excavating a 180 kilometre crater in sulfate rich rock and lofting aerosols that produced a global impact winter. Roughly 76 percent of species vanished, including every non avian dinosaur, and the iridium anomaly at the base of the boundary clay at El Kef in Tunisia is the GSSP for the Danian that follows.

Cenozoic Era, 66 to 0 Ma

The Cenozoic covers the last 66 million years and it is by far the best resolved part of the chart, which is why it looks so crowded. Its overwhelming theme is cooling. The era begins in a greenhouse with no ice anywhere and ends with ice sheets on both poles and orbital cycles driving the climate, and almost every major biological change in it is downstream of that.

Paleogene Period, 66 to 23.04 Ma

The Paleogene runs from the extinction to the start of the Miocene and holds the hottest and one of the coldest transitions of the last 100 million years. It contains three epochs and the shift from a fully ice free world to a glaciated Antarctica happens inside it.

Paleocene Epoch, 66 to 56 Ma

The Paleocene is the recovery. With the dinosaurs gone, surviving mammals expanded into empty niches and body sizes rose by around three orders of magnitude within ten million years. Most of them are still archaic groups that have no living descendants. Climate warmed steadily through the epoch, and it ends with the sharpest carbon release in the Cenozoic.

Eocene Epoch, 56 to 33.9 Ma

The Eocene opens with the Paleocene Eocene Thermal Maximum, and the GSSP at Dababiya in Egypt is placed at the base of the carbon isotope excursion that marks it. Thousands of gigatonnes of carbon entered the ocean and atmosphere within about 20 thousand years, global temperature rose 5 to 8 degrees Celsius, deep sea carbonate dissolved, and 30 to 50 percent of benthic foraminiferal species went extinct. It is the closest natural analogue we have to a rapid carbon release and it is studied intensively for that reason.

The rest of the epoch is a long cooling from that peak. Crocodilians and palms lived inside the Arctic Circle early on. Primates appear on three continents right at the base. By the end, the opening of Drake Passage and the Tasmanian gateway allowed the Antarctic Circumpolar Current to form, thermally isolating Antarctica, and at the Oi 1 event benthic oxygen isotopes jump about 1.5 per mil within 400 thousand years as continental ice appears.

Oligocene Epoch, 33.9 to 23.04 Ma

The Oligocene is the first icehouse epoch of the Cenozoic. Its base at Massignano in Italy is defined on the last appearance of the foraminiferal genus Hantkenina. Sea level fell as water locked into Antarctic ice, continental interiors dried, and open habitats spread at the expense of forest. In Europe the Grande Coupure replaced the archaic endemic mammal fauna with Asian immigrants arriving across newly emergent land. The Ethiopian and Afar flood basalts erupted at about 30 Ma and started the Red Sea, the Gulf of Aden and the East African Rift.

Neogene Period, 23.04 to 2.58 Ma

The Neogene is where the modern world takes shape: the continents reach nearly their present positions, grasslands spread across the drying subtropics, and the Northern Hemisphere starts to freeze at the end.

Miocene Epoch, 23.04 to 5.333 Ma

The Miocene is the longest epoch of the Neogene and it contains both a warm peak and a hard cooling. The Mid Miocene Climatic Optimum between about 16.9 and 14.7 Ma ran 3 to 4 degrees Celsius above present, with the Columbia River Basalt Group venting carbon dioxide at the same time. After it, the East Antarctic Ice Sheet became permanent.

The biological signature is grass. Grasses using C4 photosynthesis, which is far more efficient at low carbon dioxide and in seasonal drought, spread across the tropics and subtropics from about 8 Ma, and grass is abrasive, so grazing mammals evolved high crowned teeth in response. Horses, antelope and their relatives all record it. The epoch closes with the Messinian Salinity Crisis, when the Mediterranean was cut off from the Atlantic and largely evaporated, laying down a kilometre of salt and lowering global ocean salinity by around 6 percent.

Pliocene Epoch, 5.333 to 2.58 Ma

The Pliocene is the last time carbon dioxide sat where it is today for a sustained period, at roughly 350 to 400 ppm, and it is therefore the standard reference point for what a warm modern world looks like: global temperature about 2 to 3 degrees Celsius above preindustrial and sea level perhaps 20 metres higher. The Isthmus of Panama shoaled and closed, ending Pacific and Atlantic exchange, strengthening the Gulf Stream and opening the Great American Biotic Interchange. In East Africa, Australopithecus was walking upright.

Quaternary Period, 2.58 to 0 Ma

The Quaternary covers only the last 2.6 million years but it gets two epochs and four stages, because the resolution available in ice cores, lake sediments and speleothems is annual. Its defining feature is the ice age cycle.

Pleistocene Epoch, 2.58 to 0.0117 Ma

The Pleistocene is not one ice age but dozens. Ice sheets advanced and retreated on a 41 thousand year rhythm early on, following the tilt of Earth axis, and then switched to a roughly 100 thousand year rhythm after about 900 ka in the Middle Pleistocene Transition, a change that is still not fully explained. At the Last Glacial Maximum around 21.5 ka, ice up to three kilometres thick covered Canada and Fennoscandia, sea level was about 125 metres lower and global mean temperature was near 6 degrees Celsius below today.

Our own genus evolved and spread inside this epoch. Homo sapiens fossils from Jebel Irhoud in Morocco date to 315 ka, and the Chibanian stage that covers much of the Middle Pleistocene was ratified in 2020 on the Matuyama to Brunhes magnetic reversal at Chiba in Japan. The epoch closes with the loss of about 65 percent of large mammal genera, with human expansion and rapid climate swings both implicated and the relative weight still argued.

Holocene Epoch, 0.0117 to 0 Ma

The Holocene begins at the end of the Younger Dryas cold snap, 11700 years ago, and its GSSP is not an outcrop at all but a depth in the NGRIP2 ice core in central Greenland. That is the only way to get the resolution the epoch needs. It is divided into three ages, each defined on a climate event: the Greenlandian from the Younger Dryas termination, the Northgrippian from the 8.2 ka cooling event, and the Meghalayan from the 4.2 ka drought, whose stratotype is a stalagmite from Mawmluh Cave in Meghalaya. We are living in the Meghalayan.

You will often see the Anthropocene added after it. As of 2026 it is not a formal unit. The proposal to define an Anthropocene Series with a GSSP in the sediments of Crawford Lake in Ontario, placing the base at about 1952 on the plutonium fallout signal, was voted down by the Subcommission on Quaternary Stratigraphy in early 2024 and the result was upheld. The term remains widely and legitimately used as an informal description of the current interval. It is simply not on the chart, which is why you will not find it in the tool above.

How to use this tool

The chart runs oldest at the bottom, youngest at the top, the way a real section stacks. Scroll the wheel on it to zoom, drag to pan, and click any coloured band to open it in the panel on the right. On a phone, pinch with two fingers to zoom and slide two fingers to pan, while a single finger scrolls the page as usual. If you would rather not gesture at all, the Navigate strip above the chart has zoom in, zoom out, younger, older and two fit buttons, and it shows the exact window you are looking at. That panel gives you the base and top ages, the duration, the position in the hierarchy, the GSSP that defines the base of the interval, and every event that falls inside it. Double click a band and the view snaps to fit it. The Jump menu is faster if you already know where you are going, and the search box takes an interval name, an event name, a GSSP locality such as Meishan or Dob's Linn, or a bare number, in which case it tells you which intervals contain that age in Ma.

The column toggles decide what is drawn. Turn off the eon and era columns when you are working inside one period and want the stage names to have room to breathe. Turn on magnetic polarity when you are correlating against a palaeomag log, and turn on the land mammal ages or the planktonic foraminiferal zones when you need to tie an ICS stage to the scheme your data is actually reported in. The overlay chips add temperature, carbon dioxide, sea level and oxygen as curves drawn against the same vertical axis, so you can read a stage boundary and a sea level fall on the same line.

The input people get wrong is the unit. A great many geological ages are quoted in ka or in years before present and then typed into a Ma box without conversion, which puts the Younger Dryas somewhere in the Silurian. The Locate an age panel has an explicit unit selector for exactly this reason. The second most common error is a sign convention: ages here run backwards from the present and are always positive, so an age of 0.0117 Ma means 11700 years ago, not 11700 years in the future.

What the chart actually is

The International Chronostratigraphic Chart is not a summary of Earth history. It is a set of definitions. Each coloured band is a chronostratigraphic unit, a body of rock formed during a particular interval of time, and the boundary between two of them is defined by a single physical point in a single outcrop somewhere on the planet. Everything else, including the numbers you see in the age column, is a measurement made afterwards.

That distinction matters more than it sounds. When the Permian and Triassic boundary moved from 251.0 Ma to 252.2 Ma and then to 251.902 Ma over three decades, the boundary itself never moved. It has sat in the same 4 cm bed at Meishan since 2001. What changed was the radiometric calibration of that bed. The definition is a rock. The age is data attached to the rock, and data gets better.

The hierarchy has five ranks in normal use. An eon contains eras, an era contains periods, a period contains epochs, and an epoch contains ages. The rock body equivalents are eonothem, erathem, system, series and stage, which is why a geologist says Upper Jurassic when talking about rocks and Late Jurassic when talking about time. The Carboniferous also carries two subsystems, the Mississippian and the Pennsylvanian, which is a compromise between the North American usage of two separate periods and the European usage of one. This tool draws them in the period column, which is where they belong.

This build carries 177 intervals: 4 eons, 10 eras, 22 periods, 2 subperiods, 34 epochs and 101 stages, plus the Pridoli, which is a series with no stages defined inside it and therefore leaves the only gap in the stage column. If you zoom into 421 Ma you will see that gap. It is not a rendering error, it is the state of the Silurian.

The chart is not a ruler

Look at the age column across the whole Phanerozoic and the first thing you should notice is how uneven it is. The Norian runs for 21.6 million years. The Greenlandian runs for 3500 years. That is a ratio of over 6000 to 1 between the longest and shortest stages on the same chart, at the same rank, drawn with the same weight of line.

The reason is not that the Late Triassic was somehow a slower time. It is that the resolution of the record improves enormously toward the present. In the Quaternary we have ice cores with annual layers, so the Holocene has been cut into three stages on the strength of two climate events 8200 and 4200 years ago. In the Late Triassic we have ammonoid zones in a handful of sections and radiometric ages with uncertainties of a few hundred thousand years, so a single stage covers a span longer than the entire Cenozoic mammal radiation.

Practically, this means you cannot treat a stage as a unit of time. Saying two events are in the same stage tells you almost nothing if that stage is the Norian, and tells you a great deal if it is the Northgrippian. When you compare rates of anything across the Phanerozoic, use the numeric ages, not the stage count. The Duration between two levels panel exists to make that easy: it reports the elapsed time between any two boundaries and, because it is genuinely hard to feel a number like 21.6 million years, it also expresses it as a fraction of Earth history and as a count of human generations.

The other thing the chart hides is that the Phanerozoic, the part that fills every textbook, is 11.8 percent of Earth history. Switch the Jump menu to All of Earth history and the entire visible fossil record collapses to a sliver at the top. The Compress Earth history panel makes the same point differently: on a calendar where the planet forms at midnight on 1 January, the Cambrian starts on 18 November, the dinosaurs die at 17:24 on 26 December, and our own species turns up at 23:23:44 on 31 December. All of recorded human history is the last tenth of a second.

What a golden spike actually is

A Global Boundary Stratotype Section and Point, universally shortened to GSSP, is a physical marker driven into a specific bed in a specific outcrop. It defines the base of a stage, and by defining the base of one stage it defines the top of the one below. There is no separate definition for the top of a unit, which is a small piece of elegance worth appreciating: it makes gaps and overlaps impossible by construction.

The requirements are strict. There must be a marker event that can be recognised on other continents, almost always the first appearance of a fossil species, because a first appearance propagates outward from wherever the species evolved and is therefore roughly synchronous. There must be continuous marine sedimentation across the level, since a hiatus would mean the point defines nothing. There must be independent ways to correlate the same level, typically volcanic ash beds for radiometric dating, a carbon isotope excursion, a magnetic reversal, or a second fossil group. And the outcrop must be permanently accessible and legally protected, because the definition is the rock and the rock has to survive.

The base of the Triassic at Meishan is the textbook case, and the diagram below dissects it. The marker is the first appearance of the conodont Hindeodus parvus at the base of bed 27c. Two altered volcanic ash beds bracket the level: bed 25 at 251.941 plus or minus 0.037 Ma and bed 28 at 251.880 plus or minus 0.031 Ma, which is how the boundary itself gets its age of 251.902 plus or minus 0.024 Ma. Those two ashes are also what tell us the main extinction pulse took 60 thousand years, plus or minus 48. And running through the whole interval is a negative carbon isotope excursion of 4 to 7 per mil, which is the signal that lets a geologist find the same level in a section that contains no conodonts at all.

Of the 101 stages on the chart, 80 now have a ratified point like this one. Seventeen have a leading candidate section but no agreed level, and four have nothing at all beyond a name and a working concept. The tool colours these honestly: click the Kungurian, the Serpukhovian, the Berriasian or Cambrian Stage 3 and the panel says candidate or not yet defined rather than pretending. If your stratigraphy depends on one of those boundaries, you are correlating against a moving target, and the honest thing to do in a report is say which chart version you used.

The recent history is worth knowing. The Aeronian and Telychian GSSPs were both replaced in 2024, moving to Hlasna Treban in the Czech Republic and El Pintado in Spain. The Valanginian was ratified at Vergol in France in December 2024. The Berriasian proposal at Tre Maroua was voted down in 2020 and the working group has since moved to Torre de Busi in Italy. A Bartonian proposal was rejected in November 2024 by 14 votes to 5. Charts printed before 2024 will disagree with this one in the Silurian, the Ordovician, the Devonian and parts of the Permian, and the disagreements are not small.

Where the numbers come from, and where they stop

Below the Cambrian the whole approach changes. There are no widely correlatable fossils in most of the Precambrian, so the boundaries are Global Standard Stratigraphic Ages, which are round numbers chosen by committee. The Tonian starts at 1000 Ma exactly. The Statherian starts at 1800 Ma exactly. These are not measurements, they are conventions, and the chart flags them as such: click any Proterozoic period in this tool and the panel says Global Standard Stratigraphic Age rather than showing a section.

There are two exceptions moving in the right direction. The Ediacaran has a real GSSP at Enorama Creek in the Flinders Ranges, placed at the base of the cap carbonate that sits directly on Marinoan glacial deposits, which is why its base is 635 Ma rather than a round 650. The base of the Archean was moved to 4031 Ma to match the age of the oldest well dated Acasta Gneiss, which is a numeric definition tied to a real rock rather than to a committee's preference for zeros. The Hadean below it remains informal, running from planetary accretion at about 4567 Ma down to whatever crust survives.

Inside the Phanerozoic the ages come from three sources working together. Radiometric dates, almost all uranium lead on zircon from volcanic ash beds, provide anchor points with uncertainties now down to a few tens of thousands of years in favourable cases. Astronomical tuning, matching sedimentary cyclicity to orbital solutions, provides continuous high precision timing over the last 50 million years and increasingly further back. Everything between anchors is interpolated, usually with a spline through biostratigraphic and magnetostratigraphic tie points. That interpolation is why boundary ages move by a few hundred thousand years between chart versions without anyone finding a new rock.

A worked check you can repeat on paper

Open the Age depth model panel. It loads with a two datum example from a hypothetical deep sea core: a nannofossil datum at 120 m assigned 5.333 Ma, which is the Miocene and Pliocene boundary, and a second datum at 470 m assigned 11.63 Ma, which is the base of the Tortonian. The question is what age sits at 295 m.

First the accumulation rate, which is just the depth interval divided by the time interval.

rate = (470 - 120) / (11.63 - 5.333)
     = 350 / 6.297
     = 55.58 m per Myr

Then the age at the target depth, interpolating linearly between the two datums.

age = 5.333 + (295 - 120) / 55.58
    = 5.333 + 175 / 55.58
    = 5.333 + 3.149
    = 8.482 Ma

The tool reports 8.482 Ma and places it in the Tortonian, which runs from 11.63 to 7.246 Ma, so the answer is comfortably inside a single stage and does not depend on which side of a boundary you fall. Note the unit identity that trips people up in the same panel: 55.58 metres per million years is exactly 55.58 millimetres per thousand years, and 5.558 centimetres per thousand years. Metres per Myr and millimetres per kyr are the same number. Centimetres per kyr are ten times smaller.

Now check a radiometric age by hand. Open the Radiometric age panel with the uranium 238 to lead 206 system and a measured ratio of 0.1. The closed system age equation is t equals ln of one plus the daughter to parent ratio, divided by the decay constant.

lambda(238U) = 1.55125e-10 per year
ln(1 + 0.1)  = 0.09531018
t = 0.09531018 / 1.55125e-10
  = 614,410,000 years
  = 614.4 Ma

That lands in the Ediacaran, at 635 to 538.8 Ma, and the tool says so. As a sanity check on the decay constant itself, ln 2 divided by 1.55125e-10 gives 4.468 billion years, which is the published half life of uranium 238 to three decimal places. Run the same check on carbon 14 and a ratio of exactly 1, meaning half the parent has gone, and you get 5730 years, the Cambridge half life, which is what the tool uses.

One number from that panel is worth carrying around. At the Cretaceous and Paleogene boundary, 66.05 Ma, the expected lead 206 to uranium 238 ratio is 0.010299. Just over one percent of the uranium in a zircon that crystallised as the Chicxulub ejecta settled has turned into lead. That is the entire signal on which the age rests, which is why sample preparation and chemical abrasion matter so much more than the arithmetic.

Both engines in this tool were written twice. A reference implementation was built and tested in Node against hand calculations and published half lives, then the browser version was checked against the same cases through a debug hook on the live page. The structural checks are stricter than the arithmetic ones: every rank is tested for gaps and overlaps, every stage is tested for containment inside exactly one epoch, one period, one era and one eon, and the durations of all 101 stages plus the Pridoli are summed and required to equal 538.8 million years exactly. That is 2821 assertions, and they all pass. The one deliberate exception is the Pridoli gap in the stage column, which is real.

Reading the overlay curves honestly

The four curves are useful and they are also the least certain thing on the page, so treat them differently from the boundary ages. Temperature comes from the PhanDA reconstruction of Judd and others in 2024, which assimilates proxy data into climate model simulations, with the Cenozoic tuned to Westerhold and others. Carbon dioxide follows Foster, Royer and Lunt with the CenCO2PIP consensus over the Cenozoic. Sea level combines Haq and Schutter for the Paleozoic with Miller and others for the Cenozoic. Oxygen follows Krause and others and the GEOCARBSULF family of models.

What that means in practice is that the shape is more trustworthy than the value. The Late Paleozoic Ice Age really is the coldest part of the Phanerozoic and the Early Triassic really is the hottest, and those relationships survive every reconstruction. The absolute number at 300 Ma does not: different methods disagree by several degrees. Oxygen is the worst of the four, because it is entirely model output rather than a measurement, and the classic Carboniferous peak near 30 percent that explains giant insects is a GEOCARBSULF result that other sulfur isotope models push down considerably.

Sea level deserves a specific warning. The curve is eustatic, meaning global mean, and no outcrop anywhere records eustasy directly. What a section records is relative sea level, which is eustasy plus local subsidence plus sediment supply plus compaction. A 30 metre rise on the curve can appear as a 200 metre transgression in a rapidly subsiding basin and as nothing at all on a stable craton. Use the curve to ask whether a flooding surface has a plausible global partner, never to date one.

Magnetic polarity, the other clock

Turn on the polarity column and zoom into the Cenozoic. The black bars are intervals of normal polarity, matching today, and the white gaps are reversed. Between now and 83.64 Ma the field spent 46.6 percent of its time normal and the rest reversed, in a pattern that looks random because it is: reversals are not periodic and the interval between them ranges from about 20 thousand years to tens of millions.

That irregularity is exactly what makes the pattern useful. A sequence of five reversals with particular relative spacings is effectively a barcode, and once you can match your section to the global polarity timescale you have a set of tie points that work anywhere on the planet, in any lithology, with no fossils required. Three of the ICS stage boundaries are defined directly on reversals: the Campanian on the base of Chron C33r, the Thanetian on the base of C26n, and the Chibanian on the Matuyama to Brunhes reversal at Chiba in Japan.

Then look at what happens above 83.64 Ma. The column goes solid black for 42 million years. The Cretaceous Normal Superchron ran from 125.93 to 83.64 Ma without a single full reversal, which strips the entire mid Cretaceous of magnetostratigraphic resolution. If you work in Aptian to Santonian rocks, you have biostratigraphy, carbon isotopes and radiometric ages, and that is the whole toolkit. Above 125.93 Ma the tool draws a hatched pattern rather than guessing, because the M sequence exists but the calibration is a different argument.

Mistakes that catch people out

Quoting a stage without quoting a chart version. Saying a sample is early Tremadocian was worth 485.4 Ma on the 2020 chart and is worth 486.85 Ma on this one. Over the same period the base of the Silurian moved from 443.8 to 443.1 Ma and the base of the Devonian from 419.2 to 419.62. If your report quotes an age to three decimal places, it needs to quote the chart edition too, and this tool will copy that citation for you.

Treating a boundary age as belonging to the older unit. A GSSP defines the base of the younger unit, so 66 Ma is the first instant of the Danian, not the last instant of the Maastrichtian. The lookup in this tool follows that convention, which is why typing 66 returns Danian and typing 66.001 returns Maastrichtian. It looks pedantic until you are counting samples either side of a boundary and your bins are off by one.

Interpolating an age depth model straight through a hiatus. Linear interpolation between two datums assumes continuous deposition. If there is a disconformity between them, and in shelf sections there usually is, the model will hand back a confident age for a level that does not exist. The tell is an accumulation rate that is much lower than the lithology can plausibly support. If a mudstone comes out at 3 metres per million years, you have found missing time, not a slow ocean.

Reporting compacted rates as depositional rates. The rate the age depth model gives you is measured on the core as it sits today, after burial. A mudstone can lose more than half its original thickness to compaction, so the rate at which sediment actually arrived on the sea floor was often two to three times higher. Decompact before you compare against a modern flux, and say which one you are quoting.

Assuming a first appearance is an evolutionary origin. The marker fossils that define GSSPs appear in the record when a species arrives in that basin and becomes abundant enough to find, which can lag its actual origination by a long way and varies from section to section. This is the Signor Lipps effect running in the other direction. It is also the main reason boundary working groups argue for decades over which of two candidate levels to pick.

Reading Precambrian boundaries as if they were dated. The Tonian does not begin at 1000 Ma because something happened at 1000 Ma. It begins there because a committee needed a boundary and picked a round number. Treating those as measured ages, and worse, propagating their apparent precision into a calculation, is a category error the chart itself warns against by drawing them differently.

Regional scales, and why your data is not in ICS units

A great deal of real stratigraphic data is not reported in ICS stages at all. Terrestrial vertebrate work in North America uses land mammal ages: Wasatchian, Bridgerian, Uintan. Cenozoic marine micropalaeontology uses planktonic foraminiferal zones and calcareous nannofossil zones. British Carboniferous work uses Pendleian and Arnsbergian. Russian and Chinese literature carries its own regional stages. These are not competitors to the ICS chart, they are higher resolution local schemes that exist because the global markers are not always present in the rocks you have.

Two of those parallel scales are drawn here as optional columns, the North American land mammal ages and the planktonic foraminiferal primary biozones, and the Locate an age panel reports both when they apply. The calibration between a regional scheme and the ICS chart is itself a research result that gets revised, so treat the alignment as approximate at the tens of thousands of years level. Use it to translate between schemes, not to inherit precision from one into the other.

What this tool does not do

It does not carry uncertainties on every boundary. The published chart gives plus or minus values for many stage bases and the honest ones range from about 0.024 million years at the Permian and Triassic boundary to several million years in the early Paleozoic. Those uncertainties are not plotted here, and the age column shows single values, so treat a quoted boundary as a best estimate rather than a hard edge. It also does not carry biozonations beyond the two included, does not reconstruct plate positions, and does not attempt the M sequence polarity calibration older than 125.93 Ma.

It does carry the whole chart offline. Everything, including all 177 intervals, the GSSP records, the events, the curves and every calculation, runs in your browser with nothing uploaded and no network call after the page loads. Save the page and it works in the field with no signal. The download buttons give you the same data as CSV or JSON if you would rather have it in a spreadsheet or a script.

Related Calculators and Reference

• Pangea Map and Continental Drift Reconstruction puts the Permian, Carboniferous and Triassic on this axis into their palaeogeography, from the Gondwanan ice sheet to the breakup of the supercontinent.
• Subduction Zone Explorer and Megathrust Calculator is where slab ages, arc initiation dates and the 1700 Cascadia earthquake come from, all of which sit on this axis.
• Geologic Age Unit Converter converts between Ma, Ga, ka and years before present, which is the step people skip before typing an age into this chart.
• Cross Section Maker turns borehole tops into a scaled section, so the stages you look up here have somewhere to hang.
• Well Log Correlation Panel is where you carry a datum between wells before you build the age depth model above.
• True Stratigraphic Thickness Calculator corrects a measured interval for dip and hole deviation, which you must do before any accumulation rate means anything.
• Stereonet handles the structural side once you know the ages and need to know whether the section is right way up.
• Outcrop Photo Annotator adds a scale bar and north arrow to a field photograph of the boundary you just dated.
• International Commission on Stratigraphy publishes the chart itself, the GSSP table and the ratification record this tool is built from.