So in the 18th and 19th century geologists started to notice similarities between rock formations across the world and link them together, resulting in the naming of our geologic eras.
At this point they could sort them and guess at relative lengths but they could not know in absolute number of years how long ago these periods occurred.
I was trying to figure out exactly when that changed. My understanding is that modern numbers were not nailed down until the 60s with radiocarbon dating?
Before the 60s did they at least have ballpark estimates?
During the Bone Wars, how old did Cope and Marsh think the dinosaurs were?
C14 is useless at those ages as the half life is too short, and it is also only useful for things that were alive. I think they estimated it by looking at sedimentation rates. Niw they use other radiometric dating.
The earliest geologists who came up with anything that could have been used to date strata were Steno and Hutton, in the 17th and 18th centuries respectively, with their ideas of superposition and uniformitarianism.
From there, it looks like the actual geological time periods were pretty much decided in the early-mid 19th century. So they knew the relative arrangement of the various periods and life forms pretty early on.
As far as accurate ages of the strata, that had to wait on radiometric dating, which is something we devised in the 20th century.
Shifting from Bishop Ussher’s biblical estimate of the Earth being created in 6006 BC to the modern 4.5 billion years was mainly due to identifying different elements that decayed over useful timescales.
Broad sequencing of geological eras was underway from about 1800, with continental scale and global sequences based on character and fossil evidence emerging around the mid-19th century. That work continues today, with the definition of the Anthropocene as the most recent [in both senses].
Timing of geological eras was guesswork but the implications of geological uniformitarianism and Darwin were that massive timescales were involved. Real numbers for most only came in the 50s-60s with the rise of radiometric dating based on measuring the steady decay of one element or isotope.
Radiocarbon - the decay of C14 carbon isotope - has a half-life of 5730 years, so is used for the past 100,000 and preferably much less than that.
I’d leave it to the rock jocks on the forum to give you the gospel, but other common forms are uranium-lead and potassium-argon dating, which optimally work on different parts of the geological timescale. There are probably many others
Once you have a dating tool its a matter of finding sites and dateable materials that tell you with certainty that you are at the upper edge of, say, the Maastrichtian age, so you can start to stick absolute dates on your succession of geological eras.
Many geologists at the time considered the Earth basically eternal, or at least were Uniformitarianists of the Hutton-Lyell school, and had worked out quite a good idea of the timeframes involved for the known stratigraphy, which was why they disagreed with Kelvin’s estimate for the age of the Earth - it very clearly disagreed with the timeline they could literally see in the rocks.
So, for instance, in John Phillips’ 1840 Life on the Earth, he gives estimates of tens of millions of years for individual formations based on stratigraphy and current rates of sedimentation.
The state of the science didn’t change much in the intervening 40 years up to the Bone Wars.
[Moderating] @Charlie_Tan , this comment does nothing to advance the discussion. Please refrain. And thank you for thinking better of it and self-reporting.
Civilization describes a complex way of life that came about as people began to develop networks of urban settlements. The earliest civilizations developed between 4000 and 3000 B.C.E., when the rise of agriculture and trade allowed people to have surplus food and economic stability.
True, archaeologist are pushing the dates back, and some earlier cities- as a center of worship get earlier. History of cities - Wikipedia.
The more complex human societies, called the first civilizations, emerged around 3000 BCE in the river valleys of Mesopotamia, Minoan Crete, the Indus Valley Civilization, China, and Egypt. An increase in food production led to the significant growth in human population and the rise of cities. The peoples of Southwest Asia and Egypt laid the foundations of Western civilization: they developed cities and struggled with the problems of organised states as they moved from individual communities to larger territorial units and eventually to empires.[9] Among these early civilizations, Egypt is exceptional for its apparent lack of big cities.[10]
Çatalhöyük is called a “proto-city” by many, for example.
Well to be fair to Kelvin scientists of the age were presented with a dilemma: there was no way the Earth could plausibly be less than at least 100 million years old, but also no way the Sun could have kept shining by any conceivable energy source for more than about 20 million years (and of course life on Earth would have to be significantly younger than that). Someone had to be very, very wrong and at the time there was no way to know who.
Even a couple of decades ago, astrophysicists faced a similar dilemma: There were techniques for determining the ages of stars, and techniques for determining the age of the Universe, but some stars appeared to be older than the Universe.
(it was the star ages that were wrong, IIRC due to misestimates of their heavy element content)
I inherited from my science-loving uncle a copy of a book called The Wonders of Science…Simplified. The book purported to be published in the US in 1949, but I strongly believe it is actually a reprinting of a British book about a decade older. They used to sell it in ads in the back pages of Popular Science and Popular Mechanics
I haven’t got my copy here, but I do recall that the temporal distances of the various geologic eras were ludicrously incorrect by modern standards. They were way too short, so that we were much closer to the age of the dinosaurs than the 66 to 250 million years ago we now reckon the Mesozoic to have been.
I don’t know if that’s what was generally believed in the 1940s (or 1930s), or if the book was simply in error relative to knowledge at the time.
The book is a hoot, an odd mixture of accurate knowledge and oddly incorrect statements. But, of course, a lot of the science books I grew up with are similarly in error these days.
And, in fact, all science is like that. Science is a continuing process. We get lazy thinking “science is always correct” but it takes a lot of refinement to actually get all the details correct.
Of course. That’s the point.
It just bothers me that the advances have taken place over my lifetime. My popular science books not only had grossly inaccurate dates for the geologic eras, they told me that Mars had canals, the moon’s craters might have been volcanic in origin, and that mountains were formed by the interior of the Earth shrinking, leaving the crust to wrinkle up like the skin of an aging apple.
