The most famous came in 1654, when Archbishop James Ussher of Ireland offered the date of 4004 B. Within decades observation began overtaking such thinking.In the 1660s Nicolas Steno formulated our modern concepts of deposition of horizontal strata. Roman poet Lucretius, intellectual heir to the Greek atomists, believed its formation must have been relatively recent, given that there were no records going back beyond the Trojan War.
He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil.Robert Hooke, not long after, suggested that the fossil record would form the basis for a chronology that would “far antedate ...From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.His geological cross-section may look something like Figure 2.Clearly, Sedimentary Rocks A were deposited and deformed before the Volcanic Dyke intruded them.
These were then eroded and Sedimentary Rocks B were deposited.By measuring the ratio of the radio isotope to non-radioactive carbon, the amount of carbon-14 decay can be worked out, thereby giving an age for the specimen in question.But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.Various geologic, atmospheric and solar processes can influence atmospheric carbon-14 levels.Since the 1960s, scientists have started accounting for the variations by calibrating the clock against the known ages of tree rings.Other, larger ones, such as Lake Winnipeg, Reindeer, Athabasca, Great Slave, and Great Bear in Canada, existed at one time but have since drained off and disappeared.