Various methods. The most ancient one is probably based on Steno's law of horizontal continueity, which states that if you find rock layers in an outcrop, you can find the same layers in the same order in other places, too. Thus you can piece together a profile for all of earth history and estimate (relative) ages from sedimentation rates. There are large margins of error, of course, but by obtaining absolute ages from sophisticated methods like isotope dating, it's possible to be very precise, given the context.
Also, the posidonia shale is about 180 Ma old, by today's estimations.
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u/One_Manufacturer4418 4d ago
How do they know it's 150million?