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We have seen that isotopic dating can be used to date the time when igneous rocks formed and when metamorphic rocks metamorphosed, but not when sedimentary rocks were deposited.So how do we determine the numerical age of a sedimentary rock?Another way to grasp the immensity of geologic time is to equate the entire 4.57 billion years to a single calendar year.
For example, work from around the world shows that the Cretaceous Period began about 145 million years ago and ended 65 million years ago.
This dated column is commonly called the geologic time scale.
During the 18th and 19th centuries, before the discovery of isotopic dating, scientists came up with a great variety of clever solutions to the question, “How old is the Earth? Lord William Kelvin, a 19th century physicist renowned for his discoveries in thermodynamics, made the most inﬂuential scientiﬁc estimate of the Earth’s age of his time.
And if a datable volcanic ash buried the strata, then the strata must be older than the ash.
rocks and sedimentary rocks or for layers of datable volcanic rocks inter-bedded with sedimentary rocks.
So the Cretaceous sandstone bed in first figure was deposited during the middle part of the Cretaceous, not at the beginning or end.